pp.c: Add dummy goto to silence -Wunused-label warning in pp_divide

The label "ret" was only referred in PERL_TRY_UV_DIVIDE block
which is not compiled on typical 32-bit build, thus it used to
trigger a warning like "label ‘ret’ defined but not used".

Added goto will be optimized out and should not incur any runtime
overhead.

pp.c

@@ -1621,6 +1621,7 @@ PP(pp_divide)
 #endif
             DIE(aTHX_ "Illegal division by zero");
         TARGn(left / right, 1);
+        goto ret;               /* redundant, but silence -Wunused-label */
     }
 
   ret: