Bluetooth Linux Monitor: fix three warnings

The warnings were:
./pcap-bt-monitor-linux.c:51:1: warning: no previous prototype for 'bt_monitor_findalldevs' [-Wmissing-prototypes]
./pcap-bt-monitor-linux.c: In function 'bt_monitor_read':
./pcap-bt-monitor-linux.c:74:9: warning: unused variable 'in' [-Wunused-variable]
./pcap-bt-monitor-linux.c: At top level:
./pcap-bt-monitor-linux.c:219:1: warning: no previous prototype for 'bt_monitor_create' [-Wmissing-prototypes]