linux-postmarketos-rockchip: Use llvm-strip for modules_install

I had originally overlooked that this was necessary. If
INSTALL_MOD_STRIP=1 is set when invoking modules_install, we need to set
LLVM=1 to ensure that llvm-strip is used instead of GNU binutils strip.

Signed-off-by: Jens Reidel <adrian@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/7361>

device/community/linux-postmarketos-rockchip/APKBUILD

@@ -2,7 +2,7 @@
 # Contributor: Quade Curry <qc8086@qc8086.com>
 pkgname=linux-postmarketos-rockchip
 pkgver=6.12.57
-pkgrel=1
+pkgrel=2
 pkgdesc="Mainline kernel for rockchip devices"
 arch="aarch64"
 _carch="arm64"
@@ -59,6 +59,7 @@ package() {
 
 	make zinstall modules_install dtbs_install \
 		ARCH="$_carch" \
+		LLVM=1 \
 		INSTALL_PATH="$pkgdir"/boot/ \
 		INSTALL_MOD_PATH="$pkgdir" \
 		INSTALL_MOD_STRIP=1 \