sysctl.c: crash due missing include

Missinc include made help printing to crash.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

sysctl.c

@@ -30,6 +30,7 @@
 #include <dirent.h>
 #include <string.h>
 #include <errno.h>
+#include <libgen.h>
 #include "proc/procps.h"
 #include "proc/version.h"