FindGTK2: Add harfbuzz to GTK2_INCLUDE_DIRS
Because gtk2 uses pango and pango needs to have harfbuzz in the include path since version 1.44 this change searches for hb.h and adds its include path to GTK2_INCLUDE_DIRS.
This fixes #19531 (closed) for my Arch Linux installation.
Fixes: #19531 (closed)
Topic-rename: FindGTK2-harfbuzz