BUILD: checks: external checker needs signal.h

check.c doesn't build on solaris since 98637e5 ("MEDIUM: Add external check")
because sigset_t is unknown. Simply include signal.h to fix the issue.

No need to backport, this is 1.6-only.

src/checks.c

@@ -15,6 +15,7 @@
 #include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>