transmission: correct gtk dependencies after NLS rework

transmission: correct gtk dependencies after NLS rework

With the recent NLS rework, BR2_ENABLE_NLS doesn't imply the absense of --disable-nls, so
configure may fail with:

configure: error: "The gtk client cannot be built without nls support.  Try adding either --enable-nls or --without-gtk"

So adjust the dependencies to use BR2_SYSTEM_ENABLE_NLS instead.

As mentioned by Arnout, an alternative solution could be to explicitly pass
--enable-nls as the stub functions are now available everywhere - But I went
with the dependency for consistency reasons.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>