tree: Add fall through annotations

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

src/bee_version_output.c

@@ -156,6 +156,7 @@ void print_format(char* s, struct beeversion *v, char *filter_pkgfullname)
                         p++;
                         continue;
                   }
+                  // fall through
                 default:
                     printf("%%%c", *p);
                     break;

src/beegetopt.c

@@ -114,6 +114,7 @@ int main(int argc, char *argv[])
        switch(opt) {
            case 'V':
                 printf("beegetopt Vx.x\n");
+                // fall through
            case 'h':
                usage();
                exit(0);