maint: remove unused include for signal.h

A number of files were including signal.h but not using any
of the functions it provides

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target-sh4/helper.c

@@ -21,7 +21,6 @@
 #include <stdio.h>
 #include <string.h>
 #include <inttypes.h>
-#include <signal.h>
 
 #include "cpu.h"