Fixed stb warning.

src/image_encode.cpp

@@ -15,6 +15,7 @@
 BX_PRAGMA_DIAGNOSTIC_PUSH();
 BX_PRAGMA_DIAGNOSTIC_IGNORED_MSVC(4100) // warning C4100: 'alloc_context': unreferenced formal parameter
 BX_PRAGMA_DIAGNOSTIC_IGNORED_MSVC(4702) // warning C4702: unreachable code
+BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wunused-parameter") // warning: unused parameter ‘alloc_context’ [-Wunused-parameter]
 #define STB_IMAGE_RESIZE_IMPLEMENTATION
 #include <stb/stb_image_resize.h>
 BX_PRAGMA_DIAGNOSTIC_POP();