intl: Don't export symbols from static MSVC .obj files.
* gettext-runtime/intl/intl-compat.c (DLL_EXPORTED): Set to empty on MSVC when
DLL_EXPORT is not defined.
* gettext-runtime/intl/printf.c (DLL_EXPORTED): Likewise.
* gettext-runtime/intl/setlocale.c (DLL_EXPORTED): Likewise.
