package/arptables: fix musl compilation issues

Fixes http://autobuild.buildroot.net/results/841/841129b8f49df205e1dabc2c2a5be70fa266768b/



Backported two patches from upstream which had solved this problem.
Please note that 0002-src-Use-stdint-types.patch is modified to apply
cleanly - details in the patch file.

Signed-off-by: Brendan Heading <brendanheading@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>