community/libcaca: sets pkgver in .pc to omit the _beta20 prefix making packages unable to build with it when they require beta 20

I've been trying to build community/mpv with libcaca support but failing even though we ship the right things for it.

Dependency caca found: NO found 0.99 but need: '>= 0.99.beta18'
Run-time dependency caca found: NO (tried pkgconfig and cmake)

This is because of https://git.alpinelinux.org/aports/tree/community/libcaca/APKBUILD#n36 where the beta20 suffix is removed from the .pc file. This is done because according to apk the version would otherwise be invalid, but now packages that actually depend on a certain beta version can not be build with it.