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

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 <redacted>
Signed-off-by: Marius Storm-Olsen <redacted>
Acked-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>