mapi: don't include X11/Xlib-xcb.h on non PTHREAD platforms

Should fix the last of the glthread build issues on windows.

src/mapi/glapi/gen/gl_marshal.py

@@ -66,10 +66,10 @@ class PrintCode(gl_XML.gl_print_base):
 
     def printRealHeader(self):
         print header
-        print '#include <X11/Xlib-xcb.h>'
-        print
         print '#ifdef HAVE_PTHREAD'
         print
+        print '#include <X11/Xlib-xcb.h>'
+        print
         print 'static _X_INLINE int safe_mul(int a, int b)'
         print '{'
         print '    if (a < 0 || b < 0) return -1;'