googletest

Fix the 'make dist' behavior to include gtest-tuple.h and gtest-tuple.h.pump.
Missing these caused failures on platforms depending on them as well as general
failures of the dedicated tests for the tuple implementation.

Change was tested by running 'make distcheck' and then extracting the result to
an entirely separate location (a subdirectory is insufficient, thank you
Autotools) and running './configure; make check'.