main/gdb: bump pkgrel to refresh caches

the 8.0 was reverted, but we need bump pkgrel since its getting rebuilt.
This is so we dont get old gdb-7.12.1-r2 from cache which does not has
same checksum in APKINDEX as the rebuild.

main/gdb/APKBUILD

 # Maintainer: Natanael Copa <ncopa@alpinelinux.org>
 pkgname=gdb
 pkgver=7.12.1
-pkgrel=2
+pkgrel=3
 pkgdesc="The GNU Debugger"
 url="https://www.sourceware.org/gdb/"
 arch="all"