cmake : fix subdir for `LLAMA_METAL_EMBED_LIBRARY` (#5985)

CMakeLists.txt

@@ -208,7 +208,7 @@ if (LLAMA_METAL)
         enable_language(ASM)
         add_compile_definitions(GGML_METAL_EMBED_LIBRARY)
 
-        set(METALLIB_SOURCE "${CMAKE_SOURCE_DIR}/ggml-metal.metal")
+        set(METALLIB_SOURCE "${CMAKE_CURRENT_SOURCE_DIR}/ggml-metal.metal")
         file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/autogenerated")
         set(EMBED_METALLIB_ASSEMBLY "${CMAKE_BINARY_DIR}/autogenerated/ggml-embed-metallib.s")