Samples: Fix no previous prototype warnings in xusb.c

Samples: Fix no previous prototype warnings in xusb.c

xusb.c:139: warning: no previous prototype for ‘display_buffer_hex’
xusb.c:169: warning: no previous prototype for ‘display_ps3_status’
xusb.c:260: warning: no previous prototype for ‘display_xbox_status’
xusb.c:281: warning: no previous prototype for ‘set_xbox_actuators’
xusb.c:299: warning: no previous prototype for ‘send_mass_storage_command’
xusb.c:354: warning: no previous prototype for ‘get_mass_storage_status’
xusb.c:401: warning: no previous prototype for ‘get_sense’
xusb.c:431: warning: no previous prototype for ‘test_mass_storage’
xusb.c:530: warning: no previous prototype for ‘read_ms_winsub_feature_descriptors’
xusb.c:583: warning: no previous prototype for ‘test_device’