pine64-pinetab2: add co-maintainership (MR 5336)

I use the PineTab 2 for various tasks on a semi-regular basis, so I
would like to co-maintain it.

---

[ci:skip-vercheck]:
Since this doesn't affect the resulting package as Co-Maintainer is a
postmarketOS schema extension that abuild doesn't read, there is no need
to bump pkgrel for this.
[ci:skip-build]: already built successfully in CI

device/testing/device-pine64-pinetab2/APKBUILD

 # Reference: <https://postmarketos.org/devicepkg>
 # Maintainer: Danct12 <danct12@disroot.org>
+# Co-Maintainer: Stefan Hansson <newbyte@postmarketos.org>
 pkgname=device-pine64-pinetab2
 pkgdesc="Pine64 PineTab 2"
 pkgver=1

device/testing/linux-pine64-pinetab2/APKBUILD

 # Reference: <https://postmarketos.org/vendorkernel>
 # Kernel config based on: arch/arm64/configs/pinetab2_defconfig
 # Maintainer: Danct12 <danct12@disroot.org>
+# Co-Maintainer: Stefan Hansson <newbyte@postmarketos.org>
 pkgname=linux-pine64-pinetab2
 pkgver=6.9.8
 _danctnix_kernel_rel=2