build: fold: fix build failure with C99

GCC 10.2 gave the following error:
"error: label at end of compound statement"

* src/fold.c (fold_file): Add a ";" to avoid C2X specific syntax.

src/fold.c

@@ -260,7 +260,8 @@ fold_file (char const *filename, size_t width)
         break;
       /* We read a full buffer of complete characters.  */
       offset_in = 0;
-    next_line:
+
+    next_line:;
     }
 
   saved_errno = errno;