tools: use program_invocation_short_name provided by libc

Thanks to Dave Reisner for pointing this out.
diff --git a/tools/rmmod.c b/tools/rmmod.c
index 7be4604..0495463 100644
--- a/tools/rmmod.c
+++ b/tools/rmmod.c
@@ -57,7 +57,7 @@
 		"\t-v, --verbose     enables more messages\n"
 		"\t-V, --version     show version\n"
 		"\t-h, --help        show this help\n",
-		binname);
+		program_invocation_short_name);
 }
 
 static int check_module_inuse(struct kmod_module *mod) {