Add subdir argument to gnome.mkenums_simple() to match install_headers()

Currently, to install my public headers and generated enums.h, I have to do something like:

includedir = join_paths(prefix, get_option('includedir'))
libblah_include_subdir = join_paths(libblah_api_name, 'libblah')
libblah_enums = gnome.mkenums_simple('enums',
  sources: libblah_headers,
  install_header: true,
  install_dir: join_paths(includedir, libblah_include_subdir),
)

# Public library bits.
install_headers(libblah_headers,
  subdir: libblah_include_subdir,
)

It would be good if gnome.mkenums_simple() took a subdir argument like install_headers, so that dealing with includedir directly was not necessary.