FindGTK2: Add harfbuzz target for dependency from pango

Since commit effafca7 (FindGTK2: Add harfbuzz to GTK2_INCLUDE_DIRS,
2019-10-01, v3.16.0-rc1~22^2), pango's dependency on harfbuzz has been
captured, but only for the `GTK2_INCLUDE_DIRS` and `GTK2_LIBRARIES`
variables.  Add the `GTK2::harfbuzz` imported target and update the
`GTK2::pango` target to depend on it.

Issue: #19531