Add __EXPORTED_HEADERS__ as predefined macro.

To weed out stuff like this in uapi/linux/types.h

ifndef __EXPORTED_HEADERS__
warning "Attempt to use kernel headers from user space, see
http://kernelnewbies.org/KernelHeaders"
endif /* __EXPORTED_HEADERS__ */

Change-Id: I6506cea6248f7a3b44a839b98e91bdd0d3a6c4cd