Apply the same C++ standard rule to the QNN examples project

examples/qualcomm is configured as its own cmake project, not as a
subdirectory of the root, so guarding the assignment is not enough there --
there is nothing to inherit from. The arm64 Windows job runs that configure
separately and its runners still compiled at 17:

    qaihub_stable_diffusion_runner.vcxproj, qaihub_llama2_7b_runner.vcxproj,
    qaihub_llama3_8b_runner.vcxproj
    safe_numerics.h(71): error C2039: 'countl_zero': is not a member of 'std'

They include the installed c10 headers, so they need the same rule the root
applies. The x64 job passed after the previous commit because it builds only
the backend, which is a subdirectory of the root configure.

Author: psiddh

examples/qualcomm/CMakeLists.txt

@@ -4,7 +4,17 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-set(CMAKE_CXX_STANDARD 17)
+# This is configured as its own project rather than from the repository root,
+# so it cannot inherit the standard and has to repeat the rule: the vendored c10
+# headers these runners include need C++20 on Windows. See the note beside
+# CMAKE_CXX_STANDARD in the root CMakeLists.txt.
+if(NOT CMAKE_CXX_STANDARD)
+  if(WIN32)
+    set(CMAKE_CXX_STANDARD 20)
+  else()
+    set(CMAKE_CXX_STANDARD 17)
+  endif()
+endif()
 # qnn_executor_runner: Like executor_runner but with QNN
 
 cmake_minimum_required(VERSION 3.19)