package/leptonica: add LEPTONICA_CPE_ID_VENDOR

cpe:2.3:a:leptonica:leptonica is a valid CPE identifier for this
package:

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



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