core,log: fix warnings with -Wswitch-default

mm-log.c: In function ‘mm_to_syslog_priority’:
  mm-log.c:82:5: warning: switch missing default case [-Wswitch-default]
     82 |     switch (level) {
        |     ^~~~~~

  mm-log.c: In function ‘log_level_description’:
  mm-log.c:118:5: warning: switch missing default case [-Wswitch-default]
    118 |     switch (level) {
        |     ^~~~~~

src/mm-log.c

@@ -88,6 +88,8 @@ mm_to_syslog_priority (MMLogLevel level)
         return LOG_INFO;
     case MM_LOG_LEVEL_ERR:
         return LOG_ERR;
+    default:
+        break;
     }
     g_assert_not_reached ();
     return 0;
@@ -124,6 +126,8 @@ log_level_description (MMLogLevel level)
         return "<info> ";
     case MM_LOG_LEVEL_ERR:
         return "<error>";
+    default:
+        break;
     }
     g_assert_not_reached ();
     return NULL;