tvheadend: add dependency on gettext (host)

tvheadend: add dependency on gettext (host)

Gettext is a prerequisite to build OpenWrt according to:
https://openwrt.org/docs/guide-developer/toolchain/install-buildsystem
but github automated tests fail without this explicit dependency:

2023-06-19T08:02:45.1940511Z   checking for py module gzip ...                   ok
2023-06-19T08:02:45.1968662Z   checking for /builder/staging_dir/host/bin/pkg-config ...ok
2023-06-19T08:02:45.1998491Z ERROR: no gettext binaries found
2023-06-19T08:02:45.1999746Z   checking for xgettext ...                         fail
2023-06-19T08:02:45.2008403Z make[2]: *** [Makefile:263: /builder/build_dir/target-arm_cortex-a9+vfpv3-d16_musl_eabi/tvheadend-2023-06-05/.configured_a17fb5ef857664f03cd0ce37cc5ea591] Error 1

Signed-off-by: Marius Dinu <m95d+git@psihoexpert.ro>