Add missing include for CMakePackageConfigHelpers #99

Job	Run time
build-and-test-windows (Debug, windows-latest)	2m 42s
build-and-test-mingw (Debug, g++, windows-latest)	2m 15s
build-and-test-unix (Debug, g++, ubuntu-latest)	1m 17s
build-and-test-mingw (Debug, clang++, windows-latest)	1m 50s
build-and-test-windows (Release, windows-latest)	2m 35s
build-and-test-unix (Debug, g++, ubuntu-20.04)	1m 17s
build-and-test-mingw (Release, g++, windows-latest)	2m 18s
build-and-test-unix (Debug, g++, macos-latest)	1m 23s
build-and-test-mingw (Release, clang++, windows-latest)	2m 0s
build-and-test-unix (Debug, clang++, ubuntu-latest)	1m 20s
build-and-test-unix (Debug, clang++, ubuntu-20.04)	1m 15s
build-and-test-unix (Debug, clang++, macos-latest)	1m 10s
build-and-test-unix (Release, g++, ubuntu-latest)	1m 46s
build-and-test-unix (Release, g++, ubuntu-20.04)	1m 53s
build-and-test-unix (Release, g++, macos-latest)	1m 10s
build-and-test-unix (Release, clang++, ubuntu-latest)	1m 43s
build-and-test-unix (Release, clang++, ubuntu-20.04)	1m 34s
build-and-test-unix (Release, clang++, macos-latest)	1m 50s
	31m 18s

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