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: 11fb60d108 ("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>

tools/testing/selftests/net/reuseport_addr_any.c

@@ -21,6 +21,10 @@
 #include <sys/socket.h>
 #include <unistd.h>
 
+#ifndef SOL_DCCP
+#define SOL_DCCP 269
+#endif
+
 static const char *IP4_ADDR = "127.0.0.1";
 static const char *IP6_ADDR = "::1";
 static const char *IP4_MAPPED6 = "::ffff:127.0.0.1";