Fix typo that prevents building when using custom tcmalloc path

Change-Id: I361da6ee6ca97ce7bf5b12241a1ff8e0924a901d
Reviewed-on: http://review.couchbase.org/30840
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>