main/gc: bump pkgrel

10b83865 (Revert "main/gc: disable-parallel-mark build option",
2020-10-30) also decreased pkgrel. For systems that already had -r2,
they won't be downgraded again, unless they provide `--available`.

Let's bump pkgrel to 3 so that those systems get the reverted version
again.

main/gc/APKBUILD

 # Maintainer: Natanael Copa <ncopa@alpinelinux.org>
 pkgname=gc
 pkgver=8.0.4
-pkgrel=1
+pkgrel=3
 pkgdesc="A garbage collector for C and C++"
 url="https://hboehm.info/gc/"
 arch="all"