Bluetooth: Host: Include full path of l2cap_br_interface.h

The header file l2cap_interface.h has been included by
tests/bsim/bluetooth/host/* cases.

To avoid potential issue, include full path of header
l2cap_br_interface.h in file l2cap_interface.h.

Signed-off-by: Lyle Zhu <lyle.zhu@nxp.com>