games-emulation/dolphin: whitelist minizip, bug #671900

games-emulation/dolphin: whitelist minizip, bug #671900

Richard reports build failure against live ebuild after
https://github.com/dolphin-emu/dolphin/commit/e6b2758ab401975c0bde2b80bec5b05485c25c5a
upstream update:
  CMake Error at CMakeLists.txt:565 (add_subdirectory):
    add_subdirectory given source "Externals/minizip" which
    is not an existing directory.

Allow using stripped down minizip.

Reported-by: Richard Yao
Closes: https://bugs.gentoo.org/671900
Package-Manager: Portage-2.3.52, Repoman-2.3.12
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>