[cmake] only use MoltenVk on Apple platforms

Signed-off-by: lizzie <lizzie@eden-emu.dev>

externals/CMakeLists.txt

@@ -393,6 +393,7 @@ if (ANDROID)
     add_library(oboe::oboe ALIAS oboe)
 endif()
 
+if (APPLE)
     # moltenvk
     if (NOT YUZU_USE_BUNDLED_MOLTENVK)
         find_library(MOLTENVK_LIBRARY MoltenVK)
@@ -406,3 +407,4 @@ if (NOT MOLTENVK_LIBRARY)
 
         set(MOLTENVK_LIBRARY "${moltenvk_SOURCE_DIR}/MoltenVK/dylib/macOS/libMoltenVK.dylib" CACHE STRING "" FORCE)
     endif()
+endif()