Fixing building on Haiku

This patch came from Haiku and was part of their GSoC project for creating a libpurple UI. The original patch can be found at https://github.com/haikuports/haikuports/commit/85fd8fe1ba987c82bc94968a82531c05247b52ad

Testing Done:
Compile only.

Reviewed at https://reviews.imfreedom.org/r/916/