glapi: Do not include dlfcn.h on Windows.

This patch fixes this MinGW build error.

  CC     glapi_gentable.lo
glapi_gentable.c:47:19: fatal error: dlfcn.h: No such file or directory

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Brian Paul <brianp@vmware.com>

src/mapi/glapi/gen/gl_gentable.py

@@ -50,7 +50,9 @@ header = """/* GLXEXT is the define used in the xserver when the GLX extension i
 #include <execinfo.h>
 #endif
 
+#ifndef _WIN32
 #include <dlfcn.h>
+#endif
 #include <stdlib.h>
 #include <stdio.h>