Build: rpm: depend on gettext-devel instead of gettext

Pacemaker's spec file calls ./autogen.sh, which calls autoreconf, which
requires the autopoint command from gettext-devel.

Whether we *should* call autogen.sh (and need BuildRequires on autoconf and
automake) is an open question. It's currently necessary, but the "clean"
solution apparently would be to create dist tarballs that include the
autotools-generated files rather than their inputs. However, downstream or
user-modified specs that patch autotools inputs would need to either add the
autogen.sh call, or modify the patches to patch the generated files instead,
which seems like creating unnecessary work for them.