virt-wireshark.m4: Defer $(prefix) substitution
    
    The autoconf manual says we shouldn't be using this variable (and
    some others) outside of Makefiles as it prevents users providing
    their own prefix at the installation phase.
    
    https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Installation-Directory-Variables.html
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>