Remove autotools leftovers from gitignore

Remove autotools leftovers from gitignore

Some tracked files were ignored due to gitignore rules that date back to
autotools based buildsystem. Update gitignore accordingly.

~# git ls-files -i --exclude-standard
Makefile
build-root/Makefile
build/external/Makefile
build/external/deb/debian/dkms/Makefile
doxygen/Makefile
extras/rpm/Makefile
src/vnet/config.h
test/Makefile
test/doc/Makefile
test/ext/Makefile

Change-Id: I3326c932cd174434211b744bf5e26bc4da450339
Signed-off-by: Benoît Ganne <bganne@cisco.com>