package/libsoil: add a comment about why we rename the Makefile

Commit 8461906d (package/libsoil: fix build using old versions of patch)
added a workaround for older patch versions, but didn't add any comment in
the .mk file explaining why this is done.

Add a comment explaining why for clarity.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>