mmtty: handle MM_LOG_LEVEL_MSG in _mm_log()

GCC 13 is unhappy about mixing enums and ints. However, if we fix the
type un mmtty's _mm_log() prototype, the compiler will find something
else to be irritiated about:

  [1/2] Compiling C object test/mmtty.p/mmtty.c.o
  ../test/mmtty.c: In function ‘_mm_log’:
  ../test/mmtty.c:283:5: warning: enumeration value ‘MM_LOG_LEVEL_MSG’ not handled in switch [-Wswitch-enum]
    283 |     switch (level) {
        |     ^~~~~~
  [2/2] Linking target test/mmtty

Fix that first.

test/mmtty.c

@@ -281,6 +281,9 @@ _mm_log (gpointer     obj,
         return;
 
     switch (level) {
+    case MM_LOG_LEVEL_MSG:
+        level_str = "message";
+        break;
     case MM_LOG_LEVEL_DEBUG:
         level_str = "debug";
         break;