libsepol: really fix static library build

In commit 52eeb04c ('libsepol: fix
static-only library build') I forgot to git add the changes made to
the libsepol.mk file itself. Those changes are needed to actually pass
STATIC=<something> when building static libraries only.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>