kbuild: __extension__ support in genksyms (fix unknown CRC warning)

Recently the __extension__ keyword has been introduced in the kernel.
Teach genksyms about this keyword so it can generate correct CRC for
exported symbols that uses a symbol marked __extension__.
For now only the typedef variant:

	__extension__ typedef ...

is supported.
Later we may add more variants as needed.

This patch contains the actual source file changes. The
following patch will hold modifications to the generated
files (*_shipped) and only after the second patch the fix
has effect.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>