afprun: Include <grp.h> for the setgroups function

This avoids a build failure with future compilers due to an implicit
function declaration.

Submitted upstream: <https://github.com/Netatalk/netatalk/pull/294>

diff --git a/etc/afpd/afprun.c b/etc/afpd/afprun.c
index 129212b7165ff108..5c2b5c8bbf5c6899 100644
--- a/etc/afpd/afprun.c
+++ b/etc/afpd/afprun.c
@@ -29,6 +29,7 @@
 #include <sys/types.h>
 /* #define __USE_GNU 1 */
 #include <unistd.h>
+#include <grp.h>
 
 #include <errno.h>
 #include <sys/wait.h>