kernel-module: fix implicit function declaration

This commit adds a header to the mix to prevent the following compile
error:

make[3]: Entering directory '/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/rtpengine-no-transcode/rtpengine-mr8.3.1.4/kernel-module'
make -C /home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/linux-4.19.122 M=/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/rtpengine-no-transcode/rtpengine-mr8.3.1.4/kernel-module O=/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/linux-4.19.122 modules
make[4]: Entering directory '/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/linux-4.19.122'
make[5]: Entering directory '/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/linux-4.19.122'
  CC [M]  /home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/rtpengine-no-transcode/rtpengine-mr8.3.1.4/kernel-module/xt_RTPENGINE.o
/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/rtpengine-no-transcode/rtpengine-mr8.3.1.4/kernel-module/xt_RTPENGINE.c: In function 'send_proxy_packet6':
/home/sk/tmp/openwrt/build_dir/target-mips_24kc_musl/linux-ath79_generic/rtpengine-no-transcode/rtpengine-mr8.3.1.4/kernel-module/xt_RTPENGINE.c:3387:14: error: implicit declaration of function 'csum_ipv6_magic'; did you mean 'csum_tcpudp_magic'? [-Werror=implicit-function-declaration]
  uh->check = csum_ipv6_magic(&ih->saddr, &ih->daddr, datalen, IPPROTO_UDP, csum_partial(uh, datalen, 0));
              ^~~~~~~~~~~~~~~
              csum_tcpudp_magic
cc1: some warnings being treated as errors

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>

kernel-module/xt_RTPENGINE.c

@@ -2,6 +2,7 @@
 #include <linux/module.h>
 #include <linux/skbuff.h>
 #include <linux/ip.h>
+#include <net/ip6_checksum.h>
 #include <linux/udp.h>
 #include <linux/icmp.h>
 #include <linux/version.h>