Include <signal.h> on gdbserver/fork-child.c (and fix regressions)

Hi,

This is another obvious patch that fixes a thinko from my previous
startup-with-shell series.  We should conditionally include <signal.h>
on gdb/gdbserver/fork-child.c because gdbserver will be putting the
inferior's terminal on the correct mode after the call to
fork_inferior, and for that it needs to ignore SIGTTOU.

This patch fixes a bunch of regressions happening on AArch64 that were
reported by Yao.

gdb/gdbserver/ChangeLog:
2017-06-09  Sergio Durigan Junior  <sergiodj@redhat.com>

	* fork-child.c: Conditionally include <signal.h>.