build-sys: add HAVE_IPPROTO_MPTCP

The QAPI schema shouldn't rely on C system headers #define, but on
configure-time project #define, so we can express the build condition in
a C-independent way.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210907121943.3498701-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>