Visual Studio Projects: Improve search for Harfbuzz headers

Look in the include/harfbuzz directory that is under the pre-defined prefix
directory, as Pango 1.44.x and later pulled in HarfBuzz headers for all builds
7 jobs for gtk-3-24 in 26 minutes and 30 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1055154
debian-meson

00:05:42

passed #1055151
fedora-autotools

00:11:33

manual #1055152
allowed to fail manual
fedora-distcheck
passed #1055153
fedora-meson

00:04:16

manual #1055156
win32-ps allowed to fail manual
msys2-mingw64-autotools
passed #1055155
win32-ps
msys2-mingw64-meson

00:02:14

 
  Docs
passed #1055157
reference

00:14:57