[build] Fix a bug for memory_gc-itest
[build] Fix a bug for memory_gc-itest

In CMakeList.txt, we should use KUDU_TCMALLOC_AVAILABLE to determine
whether tcmalloc is enabled instead of TCMALLOC_ENABLED.

Change-Id: I2982af9c7070ffd0f53791d4960c3bccfee4a8cc
Reviewed-on: http://gerrit.cloudera.org:8080/14415
Tested-by: Kudu Jenkins
Reviewed-by: Alexey Serbin <864f6b82bd94e8dda64141d4274519ee66e0102e@cloudera.com>

helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu,helifu/kudu