cta/CTA#877 ObjectStore.RetrieveQueueAlgorithms unit tests fails or succeeds base on version of cmake

As suggested by Michal I've added the missing declaration of the
full specialised getElementSummary() method.  This fixes the
problem.  Where there were two "defined" symbols now there is
only one:

[itctabuild02] CTA (master) > nm /lib64/libctaobjectstoreunittests.so.0 | grep _ZN3cta11objectstore15ContainerTraitsINS0_13RetrieveQueueENS0_23RetrieveQueueToTransferEE17getElementSummaryERKNS4_13PoppedElementE
                 U _ZN3cta11objectstore15ContainerTraitsINS0_13RetrieveQueueENS0_23RetrieveQueueToTransferEE17getElementSummaryERKNS4_13PoppedElementE
[itctabuild02] CTA (master) > nm /lib64/libctaobjectstore.so.0 | grep _ZN3cta11objectstore15ContainerTraitsINS0_13RetrieveQueueENS0_23RetrieveQueueToTransferEE17getElementSummaryERKNS4_13PoppedElementE
000000000047afa8 T _ZN3cta11objectstore15ContainerTraitsINS0_13RetrieveQueueENS0_23RetrieveQueueToTransferEE17getElementSummaryERKNS4_13PoppedElementE
[itctabuild02] CTA (master) >