Overloaded call to buildMessage

[zacnelson]

Discussion on the Google group: https://groups.google.com/forum/?pli=1#!topic/openxc/RDy7XYZOEKA

Kyle Kelly found a bug in the buildMessage function here: https://github.com/openxc/vi-firmware/blob/master/src/can/canwrite.cpp#L12

This function is called after a custom encoder is used. The custom encoder returns a uint64_t value, but buildMessage and the used bitfiled-c functions only accept type int. Here's where buildMessage is called after a custom encoding is completed: https://github.com/openxc/vi-firmware/blob/master/src/can/canwrite.cpp#L131