CMake: Make benchmarks buildable standalone


Add the necessary project file stanza that allows to configure the
benchmarks with qt-cmake-private.

Change-Id: I72f51e6779791a590de5d10635aef0588388cb04
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
commite846a08f8316b405389575395b3a4a7e0604e55e
parentd728683dc786384adf624c19d33141acf87827b8
authorJoerg Bornemann <joerg.bornemann@qt.io>
committerJoerg Bornemann <joerg.bornemann@qt.io>
commit timeWed, 30 Sep 2026 13:31:46 +0000