vim: add dependency to gettext if locale support is requested

Until now, if locale support was requested, vim would be built with NLS or
not depending on the build order. This patch now makes the dependency
explicit.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>