package/exim: add EXIM_CPE_ID_VENDOR

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

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

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

package/exim/exim.mk

@@ -9,6 +9,7 @@ EXIM_SOURCE = exim-$(EXIM_VERSION).tar.xz
 EXIM_SITE = https://ftp.exim.org/pub/exim/exim4
 EXIM_LICENSE = GPL-2.0+
 EXIM_LICENSE_FILES = LICENCE
+EXIM_CPE_ID_VENDOR = exim
 EXIM_DEPENDENCIES = host-berkeleydb host-pcre pcre berkeleydb host-pkgconf
 
 # Modify a variable value. It must already exist in the file, either