bastring declares the prototype for atomic_add, as it can't rely on
haiku-specific headers. Have this declaration match the modified
prototype from Scheduler branch merge.
Matches commit 73ad2473e7874b3702cf5b0fdf4c81b747812ed9 on Haiku side.
These are part of the C++ standard, I don't know how they went missing
in our version.
http://www.cplusplus.com/reference/std/complex/
They are needed to build MathGL library, and likely other stuff.