cmake: Install the well-known types alias headers for framework builds

qtbase commit 8d4d1c58cd37939f906edcffd22c93c479d90ce9 (QTBUG-141360)
stopped exposing the framework's Headers directory on a module's
include paths. Installed consumers now resolve non-framework-style
includes through include/QtProtobufWellKnownTypes, which holds the
syncqt forwarding headers.

The google/protobuf/*.qpb.h alias headers were only installed there
for non-framework builds; framework builds copied them solely into
the framework bundle, which is no longer on the include path. As a
result, generated code containing
    #include "google/protobuf/timestamp.qpb.h"
no longer compiles against an installed framework build of Qt, which
broke the standalone tests on all macOS CI configurations.

Install the aliases unconditionally. They resolve relative to the
flat headers next to them, which for framework builds are syncqt's
forwarding headers into the framework. The bundle copies stay so the
framework remains self-contained.

Also give the bundle copy rule a real dependency: it referenced the
undefined variable in_file_path, leaving the command without a
dependency edge to the alias it copies.

Pick-to: 6.12
Task-number: QTBUG-141360
Change-Id: I3f8a3826fad63219c6018aad662ad09ee854c2e3
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>