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

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>