Fix include location based on the include directories passed via CFLAGS.

--- afflib-3.6.12/pyaff/pyaff.c.orig
+++ afflib-3.6.12/pyaff/pyaff.c
@@ -21,7 +21,7 @@
  ****************************************************/
 
 #include "Python.h"
-#include "lib/afflib.h"
+#include "afflib.h"
 
 #include <string.h>
 #include <stdlib.h>