.gitignore: ignore spl/ and tpl/ directories except spl/Makefile

Before this commit, output files under tpl/ directry
were not ignored.
This commit fixes this problem.

And we have only one source file under spl/ directory:
 spl/Makefile

So, we can describe .gitignore more simply.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>