package/libid3tag: add .pc file and install to staging hook

package/libid3tag: add .pc file and install to staging hook

The MPD project dropped autotools support in version 0.21.x in favor of
meson. While adapting the package to the meson build infrastructure, the
recognition of libid3tag failed, as only pkg-config is used to detect
the library. Note, that the version bump of the mpd package to 0.21.x is
not submitted, yet.

To help finding the build system to detect libid3tag with pkg-config
properly, add a .pc file and install it to staging.

This is exactly what Debian and Fedora do as well.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>