tools/ccanlint: Add missing header file

tools/ccanlint/async.c uses kill(2), but doesn't include the signal.h
header it comes from.  One some platforms we get away with this via
indirect includes, but not on all.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

tools/ccanlint/async.c

@@ -6,6 +6,7 @@
 #include <sys/wait.h>
 #include <sys/stat.h>
 #include <fcntl.h>
+#include <signal.h>
 #include <stdlib.h>
 #include <unistd.h>
 #include <err.h>