build: fix build when using handy as subproject
When using libhandy as a subproject, we use its libhandy_dep
variable,
which doesn't contain the VAPI. Normally, this would be handled by
pkg-config, but since that's not used here, make sure we add it
ourselves.
Otherwise we would get errors like the following:
../src/contacts-window.vala:20.7-20.9: error: The namespace name `Hdy' could not be found
using Hdy;
^^^
../src/contacts-window.vala:34.11-34.18: error: The type name `TitleBar' could not be found
private TitleBar titlebar;
^^^^^^^^
../src/contacts-window.vala:62.11-62.21: error: The type name `HeaderGroup' could not be found
private HeaderGroup header_group;
^^^^^^^^^^^
Compilation failed: 3 error(s), 0 warning(s)