Use dissector_add_uint instead of dissector_add

[legoscia]

dissector_add_uint was introduced in Wireshark 1.6.0, with dissector_add remaining as a compatibility macro. dissector_add was finally removed in 1.6.11.

That means that while this patch fixes compilation with 1.6.11 and newer, it breaks compatibility with versions earlier than 1.6.0. Would that be acceptable?

This closes CPqD/ofdissector#2.

[greenpau]

@alnvdl , do you mind merging this PR? 😄

[greenpau]

For the reference: https://github.com/CPqD/ofdissector/issues/14