Libhandy include

Libhandy packageconfig file defines -I${includedir}/libhandy-0.0, but doesn't define a toplevel include, causing includes with libhandy-0-0/ in the path to fail.

Tested on Fedora, by installing libhandy into a prefix and building phosh

PKG_CONFIG_PATH=../../local/share/pkgconfig:../../local/lib/pkgconfig:../../local/lib64/pkgconfig meson . ../../phosh
ninja