musl-locales: explicitly depend on gettext-native

musl-locales: explicitly depend on gettext-native

The gettext class is designed for use by applications with optional NLS
support: depending on the value of USE_NLS it will either depend on
gettext-minimal-native (just the autoconf macros) or gettext-native, and
will pass --enable-nls/--disable-nls as appropriate.

However, musl-locales specifically needs to run msgfmt at runtime,
so explicitly depend on gettext-native instead.

Also remove the redundant EXTRA_OECMAKE assignment.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>