toolchain: BR2_NEEDS_GETTEXT must be y when uClibc is used

BR2_NEEDS_GETTEXT was already set to y for Buildroot internal
toolchain and external toolchains based on uClibc, but wasn't set for
toolchains based on uClibC built by the Crosstool-NG backend.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>