add #include <signal.h> for OpenBSD

We have to take care of signal(2) behavior, BSD or SysV.
But modern Linux (at least glibc-2.20 and later) works as BSD signal
so this fix did not used with sigaction(2).

Author: jg1uaa

src/main.c

@@ -14,6 +14,7 @@
 #include <math.h>
 #include <stdio.h>
 #include <stdlib.h>
+#include <signal.h>
 #include <string.h>
 #include <sys/time.h>
 #include <sys/wait.h>