selftests/net: add definition for SOL_DCCP to fix compilation errors for old libc

Many systems build/test up-to-date kernels with older libcs, and
an older glibc (2.17) lacks the definition of SOL_DCCP in
/usr/include/bits/socket.h (it was added in the 4.6 timeframe).

Adding the definition to the test program avoids a compilation
failure that gets in the way of building tools/testing/selftests/net.
The test itself will work once the definition is added; either
skipping due to DCCP not being configured in the kernel under test
or passing, so there are no other more up-to-date glibc dependencies
here it seems beyond that missing definition.

Fixes: 11fb60d1 ("selftests: net: reuseport_addr_any: add DCCP")
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>