Bug 1742900 - Make sure artifact builds include RNP shared library. r=darktrojan

MZLA_LIBRNP won't be set for artifact builds because of
--disable-compile-environment. Set it for packaging purposes.

Differential Revision: https://phabricator.services.mozilla.com/D132103

mail/installer/Makefile.in

@@ -62,6 +62,7 @@ endif
 
 ifdef MOZ_ARTIFACT_BUILDS
 DEFINES += -DMOZ_ARTIFACT_BUILDS=1
+DEFINES += -DMZLA_LIBRNP=1
 endif
 
 DEFINES += -DJAREXT=