cmake: should use the value of GPERFTOOLS_LIBRARIES as REQUIRED_VARS

cmake: should use the value of GPERFTOOLS_LIBRARIES as REQUIRED_VARS

otherwise it's always true, even if the libaries are not found. also
the cmake output will be wrong without this change:

without this change, it is:

-- Found gperftools: GPERFTOOLS_TCMALLOC_LIBRARY;GPERFTOOLS_TCMALLOC_MINIMAL_LIBRARY;GPERFTOOLS_PROFILER_LIBRARY (found version "2.5.93")

with this change, it would be:

-- Found gperftools: /tmp/gperf/lib/libtcmalloc.so (found version "2.5.93")

Signed-off-by: Kefu Chai <kchai@redhat.com>