meson: fix installation of man files
meson: fix installation of man files

install_man doesn't work with custom_targets, per
https://github.com/mesonbuild/meson/issues/1550. Go back to using
install and install_dir in the custom target.

Reviewed-by: Eric Engestrom <de0b63cf2dfb1fae8ed3c3b4244856a05677d4f0@intel.com>

waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle,waffle-gl/waffle