hidl-gen: fix help menu output

should print help menu if just `hidl-gen` is run

Test: manual
Change-Id: I8dfcade8a205fb832299c81ee3b7df88b35c2b6d
diff --git a/main.cpp b/main.cpp
index cbd0708..e7c308a 100644
--- a/main.cpp
+++ b/main.cpp
@@ -1286,6 +1286,11 @@
     const char *me = argv[0];
     OutputHandler *outputFormat = nullptr;
 
+    if (argc == 1) {
+        usage(me);
+        exit(1);
+    }
+
     int res;
     while ((res = getopt(argc, argv, "hp:o:r:L:")) >= 0) {
         switch (res) {