Fix incorrect if comparison in build_examples

[mjcarroll]

🦟 Bug fix

Summary

Introduced in #354

While I tested this with my local Windows machine, the buildfarm seems to be slightly different yielding an error.

CMake Error at C:/J/workspace/ign_common-pr-win/ws/install/gz-cmake3/share/cmake/gz-cmake3/cmake3/GzBuildExamples.cmake:54 (if):
  if given arguments:

    "C:\\J\\workspace\\ign_common-pr-win\\ws\\install\\gz-math7" "C:\\J\\workspace\\ign_common-pr-win\\ws\\install\\gz-utils2" "C:\\J\\workspace\\ign_common-pr-win\\ws\\install\\gz-cmake3"

  Unknown arguments specified
Call Stack (most recent call first):
  CMakeLists.txt:164 (gz_build_examples)

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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