package/vsftpd: set VSFTPD_CPE_ID_VALID

cpe:2.3:a:vsftpd_project:vsftpd is a valid CPE identifier for this
package:

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



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>