tcp: make tcp_build_frag() static

After the previous patch the mentioned helper is
used only inside its compilation unit: let's make
it static.

RFC -> v1:
 - preserve the tcp_build_frag() helper (Eric)

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>