FS-6293: fix mod_cdr_mongodb srcdir build

src/mod/event_handlers/mod_cdr_mongodb/Makefile.am

 include $(top_srcdir)/build/modmake.rulesam
 MODNAME=mod_cdr_mongodb
-MONGODB_DRIVER=./driver/src
+MONGODB_DRIVER=$(switch_srcdir)/src/mod/event_handlers/mod_cdr_mongodb/driver/src
 mod_LTLIBRARIES = mod_cdr_mongodb.la
 mod_cdr_mongodb_la_SOURCES  = mod_cdr_mongodb.c ./driver/src/encoding.c ./driver/src/env_posix.c ./driver/src/bson.c ./driver/src/md5.c ./driver/src/mongo.c ./driver/src/numbers.c
 mod_cdr_mongodb_la_CFLAGS   = $(AM_CFLAGS)