nptl: get rid of the last preprocessor warning when __ASSUME_TGKILL is not defined

A missing change in the previous commit.

Signed-off-by: Carmelo Amoroso <carmelo.amoroso@st.com>

libpthread/nptl/allocatestack.c

@@ -993,7 +993,7 @@ setxid_signal_thread (struct xid_command *cmdp, struct pthread *t)
 
   int val;
   INTERNAL_SYSCALL_DECL (err);
-#if __ASSUME_TGKILL
+#if defined (__ASSUME_TGKILL) && __ASSUME_TGKILL
   val = INTERNAL_SYSCALL (tgkill, err, 3, THREAD_GETMEM (THREAD_SELF, pid),
 			  t->tid, SIGSETXID);
 #else