Add generated libvirt_admin.syms into .gitignore
    
    Commit a2c5d16a70a6161449c687be74db2813b362cf5e switched to generating
    libvirt_admin.syms, but forgot to add the generated file into
    .gitignore, hence causing tree pollution post-build.
    
    Signed-off-by: Martin Kletzander <mkletzan@redhat.com>