Fix build warnings

Fix build warnings

Fix various minor build warnings:

  lib/pb-protocol/pb-protocol.c:72: warning: comparison between signed and unsigned
  lib/pb-protocol/pb-protocol.c:78: warning: comparison between signed and unsigned
  lib/pb-protocol/pb-protocol.c:141: warning: unused parameter 'buf_len'
  lib/pb-protocol/pb-protocol.c:241: warning: comparison between signed and unsigned
  discover/pb-discover.c:14: warning: no previous prototype for 'sigint_handler'
  discover/pb-discover.c:13: warning: unused parameter 'signum'
  discover/log.c:22: warning: no previous prototype for 'pb_log_set_stream'
  discover/discover-server.c:159: warning: no previous prototype for 'discover_server_notify_add'
  discover/discover-server.c:169: warning: no previous prototype for 'discover_server_notify_remove'
  discover/discover-server.c:179: warning: no previous prototype for 'discover_server_set_device_source'
  discover/discover-server.c:184: warning: no previous prototype for 'discover_server_init'
  discover/discover-server.c:229: warning: no previous prototype for 'discover_server_destroy'
  discover/device-handler.c:395: warning: comparison between signed and unsigned
  discover/paths.c:44: warning: comparison between signed and unsigned

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>