virSecuritySELinuxSetFileconHelper: Fix build with broken selinux.h
    
    There are still some systems out there that have broken
    setfilecon*() prototypes. Instead of taking 'const char *tcon' it
    is taking 'char *tcon'. The function should just set the context,
    not modify it.
    
    We had been bitten with this problem before which resulted in
    292d3f2d and subsequently b109c09765. However, with one my latest
    commits (4674fc6afd6d) I've changed the type of @tcon variable to
    'const char *' which results in build failure on the systems from
    above.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>