compat/w32dlfcn: add missing config.h include

Fixes warnings about undefined HAVE_WINRT
Signed-off-by: James Almer <jamrial@gmail.com>

compat/w32dlfcn.h

@@ -21,6 +21,7 @@
 
 #ifdef _WIN32
 #include <windows.h>
+#include "config.h"
 #if (_WIN32_WINNT < 0x0602) || HAVE_WINRT
 #include "libavutil/wchar_filename.h"
 #endif