package/exim: fix SELinux

Build is broken since commit 6aeee4007322574083533a71c2be9a74e2faed55
because services/mta module is mandatory with exim

Fixes:
 - http://autobuild.buildroot.org/results/7032d13c4a1b5979f6737d1482a8452e26c705aa

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

package/exim/exim.mk

@@ -10,7 +10,7 @@ EXIM_SITE = https://ftp.exim.org/pub/exim/exim4
 EXIM_LICENSE = GPL-2.0+
 EXIM_LICENSE_FILES = LICENCE
 EXIM_CPE_ID_VENDOR = exim
-EXIM_SELINUX_MODULES = exim
+EXIM_SELINUX_MODULES = exim mta
 EXIM_DEPENDENCIES = host-berkeleydb host-pcre pcre berkeleydb host-pkgconf
 
 # Modify a variable value. It must already exist in the file, either