linux-purism-librem5: add pmb:kconfigcheck-librem5 (MR 5633)

Follow up to MR 5562, which introduced category:librem5. Add the related
option, so CI actually enforces this category for the Librem 5.

[ci:skip-vercheck]: no need to rebuild for this

device/main/linux-purism-librem5/APKBUILD

@@ -19,6 +19,7 @@ license="GPL-2.0-only"
 options="!strip !check !tracedeps
 	pmb:cross-native
 	pmb:kconfigcheck-community
+	pmb:kconfigcheck-librem5
 	"
 makedepends="
 	bash