kernel: connmark set-dscpmark fix build on 5.4

kernel: connmark set-dscpmark fix build on 5.4

Fix header change that was done for kernel but 4.19 got missed for 5.4.

Solves nasty errors like:

8.4.0_musl/include/linux/netfilter/xt_CONNMARK.h:5,
                from connmark_listener.c:30:
/builder/shared-workdir/build/sdk/staging_dir/toolchain-aarch64_cortex-a53_gcc-8.4.0_musl/include/linux/netfilter/xt_connmark.h:23:2: error: enumerator value for 'XT_CONNMARK_VALUE' is not an integer constant
 XT_CONNMARK_VALUE = BIT(0),
 ^~~~~~~~~~~~~~~~~
/builder/shared-workdir/build/sdk/staging_dir/toolchain-aarch64_cortex-a53_gcc-8.4.0_musl/include/linux/netfilter/xt_connmark.h:25:1: error: enumerator value for 'XT_CONNMARK_DSCP' is not an integer constant
};

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>