bfd: Define ___lc_codepage_func prototype for older MinGW-w64

bfd: Define ___lc_codepage_func prototype for older MinGW-w64

In commit 68e80d96a84282d547f3b3c1234c99009521630c, the usage of
___lc_codepage_func was introduced to determine the current encoding.

Prior to version 9.0 of MinGW-w64, the function prototype for
___lc_codepage_func was missing and trying to build BFD caused the
following error:

error: implicit declaration of function ‘___lc_codepage_func’

This changeset adds a conditonal definition of
___lc_codepage_func to allow a sucessful build with MinGW-w64.

Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>