Fix compilation on Mac OS X (and probably other *BSD)

http://seclists.org/nmap-dev/2014/q2/404

Introduced in r32873, which failed to include signal.h for sigaction and
friends. Doesn't break Linux because sys/wait.h is included. POSIX
(http://pubs.opengroup.org/onlinepubs/007904975/basedefs/sys/wait.h.html)
says "Inclusion of the <sys/wait.h> header *may* also make visible all
symbols from <signal.h> and <sys/resource.h>." (emphasis mine.)