CodingStyle: Add a chapter on conditional compilation

Document several common practices and conventions regarding conditional
compilation, most notably the preference for ifdefs in headers rather
than .c files.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>