Fixed some GCC7 warnings.

src/image_decode.cpp

@@ -57,6 +57,8 @@ BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Warray-bounds");
 #if BX_COMPILER_GCC >= 60000
 BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Wmisleading-indentation");
 BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Wshift-negative-value");
+#elif BX_COMPILER_GCC >= 70000
+BX_PRAGMA_DIAGNOSTIC_IGNORED_GCC("-Wimplicit-fallthrough");
 #endif // BX_COMPILER_GCC >= 60000_
 #define STBI_MALLOC(_size)        lodepng_malloc(_size)
 #define STBI_REALLOC(_ptr, _size) lodepng_realloc(_ptr, _size)