netfilter: add missing dependency for kernel 4.14

Since kernel 4.14.75 commit ("netfilter: xt_cluster: add dependency on conntrack module")
a dependency is required on kmod-nf-conntrack.

It seems this was already present for kmod-ipt-clusterip
but not yet for kmod-ipt-cluster

Add it fixing a build error when including kmod-ipt-cluster:

Package kmod-ipt-cluster is missing dependencies for the following libraries:
nf_conntrack.ko
modules/netfilter.mk:665: recipe for target '/mnt/ramdisk/koen/firmware/builds/openwrt/bin/targets/cns3xxx/generic/packages/kmod-ipt-cluster_4.14.75-1_arm_mpcore_vfp.ipk' failed
make[3]: *** [/mnt/ramdisk/koen/firmware/builds/openwrt/bin/targets/cns3xxx/generic/packages/kmod-ipt-cluster_4.14.75-1_arm_mpcore_vfp.ipk] Error 1
make[3]: Leaving directory '/mnt/ramdisk/koen/firmware/builds/openwrt/package/kernel/linux'
Command exited with non-zero status 2
time: package/kernel/linux/compile#1.80#0.05#2.07
package/Makefile:107: recipe for target 'package/kernel/linux/compile' failed
make[2]: *** [package/kernel/linux/compile] Error 2
make[2]: Leaving directory '/mnt/ramdisk/koen/firmware/builds/openwrt'
package/Makefile:103: recipe for target '/mnt/ramdisk/koen/firmware/builds/openwrt/staging_dir/target-arm_mpcore+vfp_musl_eabi/stamp/.package_compile' failed
make[1]: *** [/mnt/ramdisk/koen/firmware/builds/openwrt/staging_dir/target-arm_mpcore+vfp_musl_eabi/stamp/.package_compile] Error 2
make[1]: Leaving directory '/mnt/ramdisk/koen/firmware/builds/openwrt'
/mnt/ramdisk/koen/firmware/builds/openwrt/include/toplevel.mk:216: recipe for target 'world' failed
make: *** [world] Error 2

Fixes: bba743458eb4 ("kernel: bump 4.14 to 4.14.75")
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
[1] https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=v4.14.75&id=b969656b46626a674232c0eadf92a394b89df07c