gettext.bbclass: also search for files in target sysroot

gettext.bbclass: also search for files in target sysroot

fwupd contains polkit policy files that it translates using polkit.its
and polkit.loc files that the next polkit release is going to
install (see https://github.com/hughsie/fwupd/issues/107).

In order to make that work with OE-core, the gettext tools must be
told to look also for files in the recipe-sysroot. Otherwise it only
uses the GETTEXTDATADIR set by the gettext-native tool wrappers, and
that only points to the files provided by gettext-native itself.

(From OE-Core rev: ebb706f9dafdbb7c4aa18e5595930bbc1a0497b1)

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>