postgresql: fix ZIC configure variable assignment

The 'ZIC=$$(ZIC)' assignment is seen as 'ZIC=$(ZIC)' by the shell, that
interprets that as command substitution causing an error like:

/bin/sh: ZIC: command not found

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/postgresql/postgresql.mk

@@ -50,7 +50,7 @@ POSTGRESQL_DEPENDENCIES += tzdata
 POSTGRESQL_CONF_OPTS += --with-system-tzdata=/usr/share/zoneinfo
 else
 POSTGRESQL_DEPENDENCIES += host-zic
-POSTGRESQL_CONF_ENV += ZIC=$$(ZIC)
+POSTGRESQL_CONF_ENV += ZIC="$(ZIC)"
 endif
 
 ifeq ($(BR2_PACKAGE_OPENSSL),y)