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

libc/kernel/tools/defaults.py

@@ -34,6 +34,7 @@ kernel_known_macros = {
     "__CHECKER__": kCppUndefinedMacro,
     "__CHECK_ENDIAN__": kCppUndefinedMacro,
     "CONFIG_X86_32": "__i386__",
+    "__EXPORTED_HEADERS__": "1",
     }
 
 # define to true if you want to remove all defined(CONFIG_FOO) tests