GH-49541: [C++] ARROW_FLIGHT_SQL=ON and ARROW_BUILD_EXAMPLES=ON need gflags

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1683,7 +1683,8 @@ endif()
 if(ARROW_BUILD_TESTS
    OR ARROW_BUILD_BENCHMARKS
    OR ARROW_BUILD_INTEGRATION
-   OR ARROW_USE_GLOG)
+   OR ARROW_USE_GLOG
+   OR (ARROW_FLIGHT_SQL AND ARROW_BUILD_EXAMPLES))
   set(ARROW_NEED_GFLAGS TRUE)
 else()
   set(ARROW_NEED_GFLAGS FALSE)