devel/gettext-tools: don't disable openmp

OpenMP is gettext was disabled 15 years ago in bfe8a9ab273f573d122fd3ae04fdd85d73d0b6e2

Building with OpenMP left for autodetection works on both powerpc (which
doesn't have libomp) and on powerpc64 (which does have libomp), so it's ok to
remove disabling it.

PR: 263634
Approved by: antoine