webkitgtk: ignore gettext when cross compiling

Unfortunately, CMake looks in CMAKE_PREFIX_PATH for binaries when
cross-compiling. This means that it will use gettext from
CMAKE_PREFIX_PATH even when we provide a gettext binary in PATH! This
is bad because the on in CMAKE_PREFIX_PATH is for the cross system,
not the native one. The only documented way I can find to change this
behavior is by manually setting the CMAKE_IGNORE_PATH variable.