FPGA: Add print statement to mem_channel tutorial to avoid cmake warning (#2322)

This change adds a print statement of the DEVICE_FLAG so that a CMake "unused variable" warning is not emitted.

Author: justin-rosner

DirectProgramming/C++SYCL_FPGA/Tutorials/Features/mem_channel/CMakeLists.txt

@@ -57,6 +57,8 @@ if(NOT DEFINED DEVICE_FLAG)
                          -DDEVICE_FLAG=Agilex7.")
 endif()
 
+message("\tDEVICE_FLAG set to ${DEVICE_FLAG}")
+
 if (NOT DEFINED PART)
     set (PART " ")
 endif()