dev-libs/ustr: Sync ebuild with upstream

Previously we were carrying a patch for https://bugs.gentoo.org/732026
which has been accepted into upstream. This also bumps the EAPI to 7.

BUG=none
TEST=sudo emerge dev-libs/ustr

Disallow-Recycled-Builds: test-failures
Change-Id: I93eeb9b08ddd83c8ea7da420a05582b43de6a687
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/2558137
Commit-Queue: Fergus Dall <sidereal@google.com>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Fergus Dall <sidereal@google.com>
Auto-Submit: Fergus Dall <sidereal@google.com>
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>