skill: use strtosig() for -l option argument

Re-enable signal number to name string, and other way around,
conversion after it broke at the time when long options where
introduced.

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

proc/libprocfs.sym

@@ -62,6 +62,7 @@ global:
 	signal_number_to_name;
 	smp_num_cpus;
 	sprint_uptime;
+	strtosig;
 	tty_to_dev;
 	unix_print_signals;
 	uptime;