package/x11vnc: set X11VNC_CPE_ID_VALID

cpe:2.3:a:x11vnc_project:x11vnc is a valid CPE identifier for this package:

  https://nvd.nist.gov/products/cpe/search/results?namingFormat=2.3&keyword=cpe%3A2.3%3Aa%3Ax11vnc_project%3Ax11vnc



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>