conntrack-tools: add patch to fix build with musl

The conntrack-tools sources use the GNU version of 'struct tcphdr',
which is not exposed by the musl headers unless _GNU_SOURCE is defined.

The included patch adds the missing definition to 'src/helpers/rpc.c'.

Fixes:

  http://autobuild.buildroot.net/results/046/04613c47b9669c28cc3ff47c65607c23730ef691/
  http://autobuild.buildroot.net/results/520/520e8f327b9bd7eea59657bad702c35c632d115d/
  http://autobuild.buildroot.net/results/3d3/3d36403ba80089faea5dd06bc7e4414d593bbfc1/


  ...

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>