compat/regex: define out variables only used under RE_ENABLE_I18N

Wrap variables that were only used RE_ENABLE_I18N in `#ifdef
RE_ENABLE_I18N`. This eliminates compiler warnings when compiling with
NO_REGEX=YesPlease.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/compat/regex/regex_internal.c b/compat/regex/regex_internal.c
index dad17a8..193854c 100644
--- a/compat/regex/regex_internal.c
+++ b/compat/regex/regex_internal.c
@@ -692,10 +692,10 @@
 	}
       else
 	{
+#ifdef RE_ENABLE_I18N
 	  /* No, skip all characters until IDX.  */
 	  int prev_valid_len = pstr->valid_len;
 
-#ifdef RE_ENABLE_I18N
 	  if (BE (pstr->offsets_needed, 0))
 	    {
 	      pstr->len = pstr->raw_len - idx + offset;