Trivial fix to .gitignore for spl/Makefile

According to the gitignore man page:

"An optional prefix "!" which negates the pattern; any matching file
excluded by a previous pattern will become included again."
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
So the directory exclude "/spl/*" must come before the exception
for spl/Makefile otherwise it has no effect.

Signed-off-by: Ralph Siemsen <ralphs@netwinder.org>
Tested-by: Masahiro Yamada <yamada.m@jp.panasonic.com> [on git v1.7.9.5 / v1.8.3.2]