linux-user/syscall.c: remove wrong forward decl of setgroups()

this declaration is wrong:
the correct prototype on linux is:
int setgroups(size_t size, const gid_t *list);

since by default musl libc exposes this symbol in unistd.h
additionally to grp.h, the wrong declaration causes a build error.

the proper fix is to simply include the correct header.

Signed-off-by: John Spencer <maillist-qemu@barfooze.de>

Reviewed-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
