Fix undeffing ARG_IS instead of ARG

Whoops.

Noticed this earlier when I was merging upstream back into VCE, and
interestingly enough, it doesn't look like cppcheck warns about
undeffing a non-existent define.

desktop_version/src/main.cpp

@@ -141,7 +141,7 @@ int main(int argc, char *argv[])
             })
         }
 #undef ARG_INNER
-#undef ARG_IS
+#undef ARG
     }
 
     if(!FILESYSTEM_init(argv[0], baseDir, assetsPath))