csound: fix cross with gettext hack
We need to ignore gettext in CMAKE_PREFIX_PATH on cross to prevent
`find_program` from picking up the wrong gettext. See the `webkitgtk`
and `fish` derivations for similar hacks (though this really should
eventually be fixed at a lower level). The wrong gettext in this case is
getting pulled in via:
csound(host) -> fluidsynth(host) -> glib(host) -> gettext(host)