gir: Add pkg-config package name and main header

Merged Tom Wagner requested to merge ryuukyu/wireplumber:gir-tweaks into master

This adds information about the name of the pkg-config package provided by wireplumber as well as which header a C program needs to include to use the library to the generated .gir file.

In particular, the pkg-config package name is needed when trying to generate rust bindings, as the generator fails without it.

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