Wrap standard headers in libresrc.h with #ifndef AGI_PRE

Originally committed to SVN as r6916.

aegisub/src/libresrc/libresrc.cpp

@@ -1,3 +1,5 @@
+#include "../config.h"
+
 #include "libresrc.h"
 
 wxBitmap libresrc_getimage(const unsigned char *buff, size_t size) {

aegisub/src/libresrc/libresrc.h

@@ -1,9 +1,11 @@
+#ifndef AGI_PRE
 #include <string>
 
 #include <wx/bitmap.h>
 #include <wx/icon.h>
 #include <wx/image.h>
 #include <wx/mstream.h>
+#endif
 
 #include "bitmap.h"
 #include "default_config.h"