linux-htc-flounder: install kernel.release again

Commit a4e9924792 ("linux-htc-flounder: Modernize package") dropped the
kernel.release install step. Since then a fresh build fails at install:

    (rootfs_htc-flounder) % mkinitfs
    ERROR: only one kernel release/flavor is supported, found: []

postmarketos-mkinitfs needs /usr/share/kernel/$_flavor/kernel.release
to detect the flavor. Restore the install line (same as other kernel
packages).

Tested: pmbootstrap 3.10.1, edge; fresh install for htc-flounder passes
mkinitfs again and the image boots on hardware.

Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8796>

device/testing/linux-htc-flounder/APKBUILD

@@ -1,7 +1,7 @@
 maintainer=""
 pkgname=linux-htc-flounder
 pkgver=6.19.0
-pkgrel=1
+pkgrel=2
 pkgdesc="Google Nexus 9 kernel fork"
 arch="aarch64"
 _carch="arm64"
@@ -50,6 +50,9 @@ package() {
 	install -Dm644 "$builddir/arch/$_carch/boot/Image.gz" \
 		"$pkgdir/boot/vmlinuz"
 
+	install -Dm644 "$builddir/include/config/kernel.release" \
+		"$pkgdir/usr/share/kernel/$_flavor/kernel.release"
+
 	make modules_install dtbs_install \
 		ARCH="$_carch" \
 		LLVM=1 \