fix compilation error due to obstack.h issue
https://bugs.gentoo.org/269638

--- a2ps-4.14/lib/obstack.h	2007-12-29 09:37:59.000000000 +0800
+++ a2ps-4.14.test/lib/obstack.h	2009-06-10 18:49:45.000000000 +0900
@@ -138,9 +138,9 @@
 #define __need_ptrdiff_t
 #endif
 #endif
+#endif
 
 #include <stddef.h>
-#endif
 
 #if defined (__STDC__) && __STDC__
 #define PTR_INT_TYPE ptrdiff_t