build: fix objcc header check

$headers is a variable set in the context of other functions (we don't
use the "local" keyword in our scripts, so those variables are global).

Currently, when checking for AVFoundation/AVFoundation.h, the actual
enabled header is math.h.

Similarly, when testing for QuartzCore/CoreImage.h, the actual enabled
header is CoreGraphics/CoreGraphics.h.

This is completely broken and may be the reason why these checks are
made in random places.
