wall: fix OSX getgrouplist, gid_t* vs int*

wall: fix OSX getgrouplist, gid_t* vs int*

This was the compiler warning:

term-utils/wall.c:156:39: warning: passing 'gid_t *const' (aka 'unsigned int *const') to
parameter of type 'int *' converts between pointers to integer types with different sign
[-Wpointer-sign]
        rc = getgrouplist(login, pw->pw_gid, buf->groups, &ngroups);
                                             ^~~~~~~~~~~
/usr/include/unistd.h:653:43: note: passing argument to parameter here
int      getgrouplist(const char *, int, int *, int *);
                                              ^

Reported-by: Ruediger Meier <ruediger.meier@ga-group.nl>
Signed-off-by: Karel Zak <kzak@redhat.com>