[Build] Only require libdispatch source on non-Darwin (#66081)

Author: drexin

stdlib/cmake/modules/StdlibOptions.cmake

@@ -247,7 +247,7 @@ if(SWIFT_ENABLE_EXPERIMENTAL_CONCURRENCY AND "${SWIFT_CONCURRENCY_GLOBAL_EXECUTO
   set(SWIFT_CONCURRENCY_USES_DISPATCH TRUE)
 endif()
 
-if(SWIFT_CONCURRENCY_USES_DISPATCH)
+if(SWIFT_CONCURRENCY_USES_DISPATCH AND NOT CMAKE_SYSTEM_NAME STREQUAL Darwin)
   if(NOT EXISTS "${SWIFT_PATH_TO_LIBDISPATCH_SOURCE}")
     message(SEND_ERROR "Concurrency require libdispatch on non-Darwin hosts.  Please specify SWIFT_PATH_TO_LIBDISPATCH_SOURCE")
   endif()