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

qemu-common.h

@@ -39,6 +39,7 @@ typedef struct Monitor Monitor;
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <assert.h>
+#include <signal.h>
 
 #ifdef _WIN32
 #include "qemu-os-win32.h"