Switch from using Copy Headers in the build stages to using Copy Files
instead.

According to the following articles, Copy Headers doesn't work with
static libraries when archiving (at least in Xcode 4).

This cset copies the files into $(BUILT_PRODUCTS_DIR)/include/$(PRODUCT_NAME),
so that they can be found whether using Debug or Archive build modes.

http://developer.apple.com/library/ios/technotes/iOSStaticLibraries/iOSStaticLibraries.pdf
http://stackoverflow.com/questions/5543854/xcode-4-cant-locate-public-header-files-from-static-library-dependancy