build: fix build with libselinux 2.3

build: fix build with libselinux 2.3

Several function signatures changed in libselinux 2.3, now taking
a 'const char *' instead of 'security_context_t'.  The latter is
defined in selinux/selinux.h as

  typedef char *security_context_t;

Signed-off-by: Eric Blake <eblake@redhat.com>
(cherry picked from commit 292d3f2d38e8faca075ababcb652f2e090b745b2)

Conflicts:
	tests/securityselinuxhelper.c - context: no commit 95577af