Log Message:
Include desktopdb.mk in packages installing .desktop files.

As of 2022-08-20, including sysutils/desktop-file-utils/desktopdb.mk
does not imply a full dependency on desktop-file-utils any more.

This makes it more feasible for smaller packages installing .desktop
files to include desktopdb.mk and thus trigger the post-install
UPDATE_DESKTOPDB hook.