LibC: Made mbtowc return int instead of size_t

Author: https://github.com/Lubrsi
Commit: db31452bc2
Pull-request: https://github.com/SerenityOS/serenity/pull/3454
Reviewed-by: https://github.com/bugaevc

Libraries/LibC/stdlib.cpp

@@ -796,7 +796,7 @@ size_t mbstowcs(wchar_t*, const char*, size_t)
     ASSERT_NOT_REACHED();
 }
 
-size_t mbtowc(wchar_t* wch, const char* data, size_t data_size)
+int mbtowc(wchar_t* wch, const char* data, size_t data_size)
 {
     // FIXME: This needs a real implementation.
     UNUSED_PARAM(data_size);