tlp-0.3.7.1.ebuild: Setting the git source to the expected version 0.3.7.1. So emerge doesn\'t fail while patching

Signed-off-by: André Erdmann <dywi@mailerd.de>
tlp-0.3.7.1: set $EGIT_COMMIT

Relevant ChangeLog entry:

   Fix building of tlp 0.3.7.1 which previously failed during src_prepare()
   because EGIT_COMMIT was not set properly. Instead, the ebuild always
   used upstream's master branch, which does no longer work since 0.3.8.1
   has has been released.

   Reported by Burak Aktas (codwow; github issue #2).

This commit adjusts the previous one [0] so that the git tag is checked out
during src_unpack() and not src_prepare().

[0] - commit 15af5c8339dd060c3e4aff3006f19bff8a00ed6dt