vsock/vmci: make vmci_vsock_transport_cb() static

Fix the following gcc-9.3 warning when building with 'make W=1':
    net/vmw_vsock/vmci_transport.c:2058:6: warning: no previous prototype
        for ‘vmci_vsock_transport_cb’ [-Wmissing-prototypes]
     2058 | void vmci_vsock_transport_cb(bool is_host)
          |      ^~~~~~~~~~~~~~~~~~~~~~~

Fixes: b1bba80a ("vsock/vmci: register vmci_transport only when VMCI guest/host are active")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>