Do not redefine WC_ERR_INVALID_CHARS

WC_ERR_INVALID_CHARS might be already defined by the Windows SDK.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Merge pull request #1668 from csware/WC_ERR_INVALID_CHARS

Do not redefine WC_ERR_INVALID_CHARS