Add missing error headers
by Pekka Helenius, 2018

--- a/sockstat.c	2008-06-01 04:52:39.000000000 +0300
+++ b/sockstat.c	2018-10-02 23:29:16.872336139 +0300
@@ -38,6 +38,7 @@
 #include <dirent.h>
 #include <ctype.h>
 #include <errno.h>
+#include <err.h>
 #include <pwd.h>
 #include <grp.h>
 #include <assert.h>