imlib2: allow compilation without x headers

Although configure can be called to ignore x headers, eventually the
compilation of api_obsolete.c fails because the configuration results
in config.h are not evaluated.

For successful compilation with `./configure --without-x` and adding
config.h I also needed stddef.h, which is added as well.

src/lib/api_obsolete.c

@@ -1,6 +1,8 @@
 /*
  * Obsolete, useless code
  */
+#include <stddef.h>
+#include "config.h"
 #include "Imlib2.h"
 
 EAPI void