Missing import in context.cpp

[eevee]

context.cpp makes use of getcwd, but is missing the associated #include <unistd.h>.

[akhleung]

Compiles fine on my test systems, but I'll add the import if it's generally expected.

[eevee]

I figured it must or someone would've noticed by now. :) I'm on Arch using g++ 4.7, if you're interested.

[akhleung]

Fixed!