librem5: Restrict dmesg access

This brings us in line with modern distros
Signed-off-by: Guido Günther <guido.gunther@puri.sm>

arch/arm64/configs/librem5_defconfig

@@ -748,6 +748,7 @@ CONFIG_ROOT_NFS=y
 CONFIG_SUNRPC_DEBUG=y
 CONFIG_NLS_CODEPAGE_437=y
 CONFIG_NLS_ISO8859_1=y
+CONFIG_SECURITY_DMESG_RESTRICT=y
 CONFIG_SECURITY=y
 CONFIG_SECURITY_APPARMOR=y
 # CONFIG_SECURITY_APPARMOR_HASH_DEFAULT is not set