[ez] Optionally link vulkan backend to devtools example runner (#13293)

Summary:
Title says it all!

cc @manuelcandales @cbilgin

Author: SS-JIA

examples/devtools/CMakeLists.txt

@@ -65,6 +65,10 @@ target_link_libraries(
   portable_kernels
 )
 
+if(EXECUTORCH_BUILD_VULKAN)
+  target_link_libraries(example_runner vulkan_backend)
+endif()
+
 if(EXECUTORCH_BUILD_COREML)
   find_library(ACCELERATE_FRAMEWORK Accelerate)
   find_library(COREML_FRAMEWORK CoreML)