Return cleanly after printing help

Test plan:
```
 $ ./build/src/bpftrace -h &> /dev/null && echo $?
 0
```

src/main.cpp

@@ -103,6 +103,9 @@ int main(int argc, char *argv[])
       case 'c':
         cmd_str = optarg;
         break;
+      case 'h':
+        usage();
+        return 0;
       default:
         usage();
         return 1;