net: Make linux/tcp.h C++ friendly (trivial)

I originally sent this patch to <trivial@kernel.org>, but Jiri Kosina did
not feel that this is fully appropriate for the trivial tree.

Using linux/tcp.h from C++ results in:

cat t.cc
#include <linux/tcp.h>
int main() { }

g++ -c t.cc

In file included from t.cc:1:
/usr/include/linux/tcp.h:72: error: '__u32 __fswab32(__u32)' cannot appear in a constant-expression
/usr/include/linux/tcp.h:72: error: a function call cannot appear in a constant-expression
...

Attached trivial patch fixes this problem.

Tested:
- the t.cc above compiles with g++ and
- the following program generates the same output before/after
  the patch:

#include <linux/tcp.h>
#include <stdio.h>

int main ()
{
#define P(a) printf("%s: %08x\n", #a, (int)a)
 P(TCP_FLAG_CWR);
 P(TCP_FLAG_ECE);
 P(TCP_FLAG_URG);
 P(TCP_FLAG_ACK);
 P(TCP_FLAG_PSH);
 P(TCP_FLAG_RST);
 P(TCP_FLAG_SYN);
 P(TCP_FLAG_FIN);
 P(TCP_RESERVED_BITS);
 P(TCP_DATA_OFFSET);
#undef P
 return 0;
}

Signed-off-by: Paul Pluzhnikov <ppluzhnikov@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>