replace SIGCLD with SIGCHLD

SIGCLD is defined with SIGCHLD anyway

see

bits/signum.h:#define     SIGCLD          SIGCHLD /* Same as SIGCHLD(System V).  */

additionally these tests can now compile on systemd with
musl libc

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/include/tst_cmd.h b/include/tst_cmd.h
index 78c1822..9f4240e 100644
--- a/include/tst_cmd.h
+++ b/include/tst_cmd.h
@@ -92,7 +92,7 @@
 }
 #endif
 
-/* Wrapper function for system(3), ignorcing SIGCLD signal.
+/* Wrapper function for system(3), ignorcing SIGCHLD signal.
  * @command: the command to be run.
  */
 int tst_system(const char *command);