Merge branch 'selinux-dlfcn-fix' into 'master'

ps/output.c: include <dlfcn.h> only when necessary

dlopen() functionality is only used when SELinux support is enabled, so
<dlfcn.h> only needs to be included when ENABLE_LIBSELINUX is
defined. This fixes the build in configurations where <dlfcn.h> is not
available.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
See merge request !21