Xcode: Fix includes in examples.

Xcode: Fix includes in examples.

The current file's directory, ".", is treated much more literally
when building libvpx examples with Xcode than it is with make, and
clang cannot find common include files included via "./" when those
files actually reside one directory up in the tree.

Change-Id: I5f66a026282e35d80248ca4052ebb882b859172e