usr/include: refactor .gitignore

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 <masahiroy@kernel.org>