build: include libmtmd in Apple XCFramework (#21935)

Adds an opt-in LLAMA_BUILD_MTMD CMake option so build-xcframework.sh
can link libmtmd.a into the framework binary without pulling in the
rest of tools/ (which doesn't cross-build cleanly to iOS/tvOS/visionOS).

- CMakeLists.txt: new option, default OFF. When on with
  LLAMA_BUILD_TOOLS=OFF, only the tools/mtmd subdir is added. Useful
  for any binding that wants just libmtmd (Apple XCFramework, WASM).
- tools/mtmd/CMakeLists.txt: gate the CLI exe targets on
  LLAMA_BUILD_TOOLS. Gating on LLAMA_BUILD_COMMON is not enough — it
  defaults ON in standalone builds and visionOS xcodebuild then fails
  with "install TARGETS given no BUNDLE DESTINATION for MACOSX_BUNDLE
  executable target 'llama-mtmd-cli'".
- build-xcframework.sh: turn the option on, pass -DLLAMA_BUILD_MTMD,
  add libmtmd.a to combine_static_libraries, and copy mtmd.h and
  mtmd-helper.h into the framework Headers dir. The umbrella module
  map then exposes them, so Swift / Obj-C consumers can import the
  mtmd C API directly.

After this, nm on ios-arm64/llama.framework/llama shows 52 _mtmd_
symbols. Verified end-to-end: a Swift target links the produced
framework and calls mtmd_default_marker, mtmd_bitmap_init, etc.
without a shim on macos / iphoneos / iphonesimulator / xros slices.

Co-authored-by: Abraham Gonzalez <abraham@theabecaster.com>

build-xcframework.sh

@@ -13,6 +13,7 @@ LLAMA_BUILD_EXAMPLES=OFF
 LLAMA_BUILD_TOOLS=OFF
 LLAMA_BUILD_TESTS=OFF
 LLAMA_BUILD_SERVER=OFF
+LLAMA_BUILD_MTMD=ON
 GGML_METAL=ON
 GGML_METAL_EMBED_LIBRARY=ON
 GGML_BLAS_DEFAULT=ON
@@ -39,6 +40,7 @@ COMMON_CMAKE_ARGS=(
     -DLLAMA_BUILD_TOOLS=${LLAMA_BUILD_TOOLS}
     -DLLAMA_BUILD_TESTS=${LLAMA_BUILD_TESTS}
     -DLLAMA_BUILD_SERVER=${LLAMA_BUILD_SERVER}
+    -DLLAMA_BUILD_MTMD=${LLAMA_BUILD_MTMD}
     -DGGML_METAL_EMBED_LIBRARY=${GGML_METAL_EMBED_LIBRARY}
     -DGGML_BLAS_DEFAULT=${GGML_BLAS_DEFAULT}
     -DGGML_METAL=${GGML_METAL}
@@ -126,6 +128,8 @@ setup_framework_structure() {
     cp ggml/include/ggml-cpu.h     ${header_path}
     cp ggml/include/ggml-blas.h    ${header_path}
     cp ggml/include/gguf.h         ${header_path}
+    cp tools/mtmd/mtmd.h           ${header_path}
+    cp tools/mtmd/mtmd-helper.h    ${header_path}
 
     # Create module map (common for all platforms)
     cat > ${module_path}module.modulemap << EOF
@@ -247,6 +251,7 @@ combine_static_libraries() {
         "${base_dir}/${build_dir}/ggml/src/${release_dir}/libggml-cpu.a"
         "${base_dir}/${build_dir}/ggml/src/ggml-metal/${release_dir}/libggml-metal.a"
         "${base_dir}/${build_dir}/ggml/src/ggml-blas/${release_dir}/libggml-blas.a"
+        "${base_dir}/${build_dir}/tools/mtmd/${release_dir}/libmtmd.a"
     )
 
     # Create temporary directory for processing