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.)

nmap_tty.cc

@@ -163,7 +163,8 @@ static void tty_flush(void)
         FlushConsoleInputBuffer(stdinput);
 }
 
-#else
+#else  //!win32
+#include <signal.h>
 #if !defined(O_NONBLOCK) && defined(O_NDELAY)
 #define O_NONBLOCK			O_NDELAY
 #endif