libselinux: adjust build of Python bindings

libselinux: adjust build of Python bindings

In order to work-around dependency issues, the Python bindings should be
built though a separate make invocation from the rest of the
library. This avoids build issues like this:

selinux_restorecon.lo: file not recognized: File truncated
[...]
collect2: error: ld returned 1 exit status
make[2]: *** [libselinux.so.1] Error 1

Fixes:

  http://autobuild.buildroot.net/results/967b74d0ae5b4b83ea2729217b005a3e1e4514d0/

Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
[Thomas: improve commit log.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>