gcc-shared-source: do not use ${S}/.. in deploy_source_date_epoch

gcc-shared-source: do not use ${S}/.. in deploy_source_date_epoch

This function is referencing '${S}/..'.
It uses ${S} only as good known directory path to start
traversing from, and it does not need it to exist or be populated.
If ${S} does not exist yet, the function will fail because
it cannot evaluate path .. from non-existing directory.

Reproducer (verified in master and kirkstone):
  bitbake gcc -c deploy_source_date_epoch
  bitbake gcc -c cleansstate
  rm -rf build/tmp
  bitbake gcc -c deploy_source_date_epoch

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>