nimble/transport: Fix compilation with TCP socket transport

[zaporozhets]

Hi,

This patch fixes host stack compilation with BLE_SOCK_USE_TCP=1. Error message below

/usr/bin/ld: libnimble.a(ble_hci_socket.c.o): in function `ble_hci_trans_ll_evt_tx':
/yoyo/mynewt-nimble/nimble/transport/socket/src/ble_hci_socket.c:730: undefined reference to `ble_hci_sock_cmdevt_tx'
/usr/bin/ld: libnimble.a(ble_hci_socket.c.o): in function `ble_hci_trans_ll_acl_tx':
/yoyo/mynewt-nimble/nimble/transport/socket/src/ble_hci_socket.c:744: undefined reference to `ble_hci_sock_acl_tx'
/usr/bin/ld: libnimble.a(ble_hci_socket.c.o): in function `ble_hci_trans_hs_cmd_tx':
/yoyo/mynewt-nimble/nimble/transport/socket/src/ble_hci_socket.c:759: undefined reference to `ble_hci_sock_cmdevt_tx'
/usr/bin/ld: libnimble.a(ble_hci_socket.c.o): in function `ble_hci_trans_hs_acl_tx':
/yoyo/mynewt-nimble/nimble/transport/socket/src/ble_hci_socket.c:773: undefined reference to `ble_hci_sock_acl_tx'

Let me know if you have any suggestions on implementation.