gettext.c: move get_preferred_languages() from http.c

Calling setlocale(LC_MESSAGES, ...) directly from http.c, without
including <locale.h>, was causing compilation warnings.  Move the
helper function to gettext.c that already includes the header and
where locale-related issues are handled.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

gettext.h

@@ -65,4 +65,6 @@ const char *Q_(const char *msgid, const char *plu, unsigned long n)
 /* Mark msgid for translation but do not translate it. */
 #define N_(msgid) msgid
 
+const char *get_preferred_languages(void);
+
 #endif