spl: common: Properly ignore spl/Makefile in .gitignore

The spl directory is ignored by git as these objects are created
during spl creation.  The only file not created is the Makefile.

This file can be modified and checked in via git.

Due to the order of rule precedence having the whole directory
ignored first then indicating not to ignore the Makefile is not correct
the message to force adding the Makefile is still shown.

So reorder the .gitignore for the Makefile and indicate that the Makefile
does not need to be ignored first and then indicate everything else in spl
should be ignored after wards.

Signed-off-by: Dan Murphy <dmurphy@ti.com>