[examples/cpp] Fix missing parentheses (#36382)

Fix https://github.com/grpc/grpc/issues/36367.

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Closes #36382

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/36382 from yijiem:fix-example-cmake 54521e87a1
PiperOrigin-RevId: 625469934

examples/cpp/keepalive/CMakeLists.txt

@@ -58,7 +58,7 @@ target_link_libraries(hw_grpc_proto
 
 # Targets greeter_[async_](client|server)
 foreach(_target
-  greeter_callback_client greeter_callback_server
+  greeter_callback_client greeter_callback_server)
   add_executable(${_target} "${_target}.cc")
   target_link_libraries(${_target}
     hw_grpc_proto