build: paste: fix build failure with C99

GCC 10.2 gave the following error:
"error: a label can only be part of a statement
 and a declaration is not a statement"

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

src/paste.c

@@ -138,7 +138,7 @@ collapse_escapes (char const *strptr)
           continue;
         }
 
-      copy_character:
+      copy_character:;
       mcel_t g = mcel_scanz (s);
       strout = mempcpy (strout, s, g.len);
       s += g.len;