linux-user: fix compile error due to stray colon at end of #ifdef line

Remove a stray colon from the end of a #ifdef line. Some versions
of gcc complain about this:
 linux-user/syscall.c: In function ‘do_syscall’:
 linux-user/syscall.c:7606:28: error: extra tokens at end of #ifdef directive [-Werror]

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-By: Riku Voipio <riku.voipio@linaro.org>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

linux-user/syscall.c

@@ -7603,7 +7603,7 @@ abi_long do_syscall(void *cpu_env, int num, abi_long arg1,
 #endif
 #else
     case TARGET_NR_sendfile:
-#ifdef TARGET_NR_sendfile64:
+#ifdef TARGET_NR_sendfile64
     case TARGET_NR_sendfile64:
 #endif
         goto unimplemented;