Work around incorrect g_file_test() behavior on Win32

g_file_test is redefined to be g_file_test_utf8 and thus can't handle
non-ASCII characters. This fix adds simple wrapper (taken from glib)
that fixes encoding and calls g_file_test_utf8. All required inclusions
of glib_compat.h are added as well.

src/decoder/wildmidi_decoder_plugin.c

@@ -19,6 +19,7 @@
 
 #include "config.h"
 #include "decoder_api.h"
+#include "glib_compat.h"
 
 #include <glib.h>