libregexp: include stddef.h in lib9.std.h

Commit 2d82ef9d98 added ptrdiff_t in regcomp.c.
However, this change broke the build of the Unix
package because ptrdiff_t is defined in stddef.h.

src/libregexp/lib9.std.h

@@ -3,6 +3,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <stddef.h>
 
 #define	exits(x)	exit(x && *x ? 1 : 0)