tools: Use POSIX signal.h instead of sys/signal.h

tools: Use POSIX signal.h instead of sys/signal.h

The POSIX spec specifies to use:
    #include <signal.h>
instead of:
    #include <sys/signal.h>
as seen here:
   http://pubs.opengroup.org/onlinepubs/009695399/functions/signal.html

This removes the warning:
    #warning redirecting incorrect #include <sys/signal.h> to <signal.h>
when building with the musl C-library.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>