build_eolian_cxx_test: Set dependencies correctly and remove BUILT_SOURCES

build_eolian_cxx_test: Set dependencies correctly and remove BUILT_SOURCES

This took quite a while to figure out. The two object names for the cxx callback
test and callback.c have been to close. callback.c needs the eolian headers and
the CXX test needs the eolian_cxx headers.

Split it up and set the deps correctly. If you always run check with something
like -j 9 you might have never noticed as it was very likely that the headers
have been generated due to the wrong dep before.