package/rpm: add gettext dependency

rpm check for libintl library if BR2_NEEDS_GETTEXT_IF_LOCALE is set.

checking for GNU gettext in libintl... yes
checking whether to use NLS... yes
checking where the gettext function comes from... external libintl
checking how to link with libintl... -lintl

But the dependency on gettext package is missing to ensures
reproducible builds.

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>