Fix compilation warning due to missing header for sigaction

Fix compilation warning due to missing header for sigaction

Fix the following warning by including signal.h directly in qemu-common.h
----8<----
iohandler.c: In function ‘qemu_init_child_watch’:
iohandler.c:172: warning: implicit declaration of function ‘sigaction’
iohandler.c:172: warning: nested extern declaration of ‘sigaction’
----8<----

Signed-off-by: Alexandre Raymond <cerbere@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>