Change regerror() declaration from K&R style to ANSI C (C89)

The MSVC headers typedef errcode as int, and thus confused the compiler in
the K&R style definition. ANSI style deconfuses it.

This patch was originally applied as v1.6.5-rc2~23 but needs to be
re-applied since compat/regex was overwritten by Ævar Arnfjörð
Bjarmason with the gawk regex engine.

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Marius Storm-Olsen <mstormo@gmail.com>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/regex/regcomp.c

@@ -546,11 +546,8 @@ weak_alias (__regcomp, regcomp)
    from either regcomp or regexec.   We don't use PREG here.  */
 
 size_t
-regerror (errcode, preg, errbuf, errbuf_size)
-    int errcode;
-    const regex_t *__restrict preg;
-    char *__restrict errbuf;
-    size_t errbuf_size;
+regerror(int errcode, const regex_t *__restrict preg,
+	 char *__restrict errbuf, size_t errbuf_size)
 {
   const char *msg;
   size_t msg_size;