Fix /CETCOMPAT being silently dropped from the user-mode binaries

The genex $<$<STREQUAL:"${WDK_PLATFORM}","x64">:/CETCOMPAT> was evaluated
before find_package(WDK) defined WDK_PLATFORM, so it froze as permanently
false. The driver got the flag back only because its own CMakeLists
re-specifies it afterwards; PawnIOLib.dll and PawnIOUtil.exe linked without
shadow stack compatibility.

Move the WDK lookup above add_link_options so the variable exists when the
genex is expanded.