The current .gitignore intends to ignore everything under usr/include/
except .gitignore and Makefile.
A cleaner solution is to use a pattern suffixed with '/', which matches
only directories. It works well here because all the exported headers
are located in sub-directories, like <linux/*.h>, <asm/*.h>.
Signed-off-by: Masahiro Yamada <firstname.lastname@example.org>