tree: Add fall through annotations

Gcc can warn on implicit fallthoughs. Mark them with a comment which is
recognized by gcc.

mx_util.c

@@ -112,17 +112,17 @@ int mx_strtobytes(char *str, unsigned long long int *bytes)
 
             case 'T': /* tebi */
                 t *= 1024;
-
+                // fall through
             case 'G': /* gibi */
                 t *= 1024;
-
+                // fall through
             case 'M': /* mebi */
                 t *= 1024;
-
+                // fall through
             case 'k': /* kibi */
             case 'K':
                 t *= 1024;
-
+                // fall through
             case 'B': /* bytes */
                 end++;
                 break;
@@ -180,19 +180,19 @@ int mx_strtoseconds(char *str, unsigned long long int *seconds)
 
             case 'y': /* years */
                 t *= 52;
-
+                // fall through
             case 'w': /* weeks */
                 t *= 7;
-
+                // fall through
             case 'd': /* days */
                 t *= 24;
-
+                // fall through
             case 'h': /* hours */
                 t *= 60;
-
+                // fall through
             case 'm': /* minutes */
                 t *= 60;
-
+                // fall through
             case 's': /* seconds */
                 end++;
                 break;

mxqsub.c

@@ -902,6 +902,7 @@ int main(int argc, char *argv[])
 
             case 2:
                 mx_log_warning("option --group_priority is deprecated. please use --group-priority instead.");
+                // fall through
             case 'P':
                 if (mx_strtou16(optctl.optarg, &arg_group_priority) < 0) {
                     mx_log_crit("--group-priority '%s': %m", optctl.optarg);
@@ -931,6 +932,7 @@ int main(int argc, char *argv[])
 
             case 4:
                 mx_log_warning("option '--time' is deprecated. please use '--runtime' or '-t' in future calls.");
+                // fall through
             case 't':
                 if (mx_strtou32(optctl.optarg, &arg_time) < 0) {
                     unsigned long long int minutes;