.gitignore: add the doxygen directory.

The doxygen directory is created by running 'make doxygen' - this
results in a huge number of new files that swamps graphical git tools.

Since this directory is a product of a build, it should be safe to
ignore.

Change-Id: I871dd2a36433d4dd46b231ebc7398e85d0278f27
Signed-off-by: Martin Roth <martin.roth@se-eng.com>
Reviewed-on: http://review.coreboot.org/7798
Tested-by: build bot (Jenkins)
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>