git: openjdk/jdk15u-dev: 8256501: libTestMainKeyWindow fails to build with Xcode 12.2

[Vladimir Kempik]

Changeset: c565a2b0
Author:    Vladimir Kempik <vkempik at openjdk.org>
Date:      2021-04-02 07:24:43 +0000
URL:       https://git.openjdk.java.net/jdk15u-dev/commit/c565a2b0

8256501: libTestMainKeyWindow fails to build with Xcode 12.2

Backport-of: 4e5116c46e29977cccbe8c04cb5559ce345fa72e

! make/test/JtregNativeJdk.gmk