Change -Wunused-but-set-variable to GCC only

Clang (as of version 10.0.0) does not support this flag.

src/image_decode.cpp

@@ -57,7 +57,7 @@ BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wint-to-pointer-cast")
 BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wmissing-field-initializers");
 BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wshadow");
 BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wsign-compare");
-BX_PRAGMA_DIAGNOSTIC_IGNORED_CLANG_GCC("-Wunused-but-set-variable");
+BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Wunused-but-set-variable");
 BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Warray-bounds");
 #if BX_COMPILER_GCC >= 60000
 BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Wmisleading-indentation");