Fix CMake-generated `libzmq.pc` file #1238

Job	Run time
build (x64, release, windows-2019, ON, ON, Visual Studio 16 2019, v142, vs2019, v142-x64, ON)	7m 51s
cron	0s
build (ubuntu-latest, default, asciidoctor, disabled, select)	2m 43s
build (ubuntu-latest, default, disabled, poll)	2m 40s
build (ubuntu-latest, android, android-ndk-r25, disabled)	3m 36s
build (ubuntu-latest, coverage, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev lcov, enabled,...	3m 23s
build (ubuntu-latest, valgrind, valgrind libgnutls28-dev, enabled)	8m 35s
build (ubuntu-latest, cmake, libsodium, enabled, cmake libsodium-dev, enabled)	4m 2s
build (ubuntu-latest, cmake, libsodium, enabled, enabled, cmake libsodium-dev libkrb5-dev, enabled)	4m 49s
build (ubuntu-latest, cmake, enabled, cmake clang-format-11, 1)	1m 17s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 55s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 29s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 33s
build (ubuntu-latest, abi-compliance-checker, abi-dumper abi-compliance-checker, disabled)	3m 22s
build (ubuntu-latest, cmake, clang-tidy clang-tools, enabled, clang++)	4m 2s
build (macos-latest, default, automake autoconf libtool, enabled)	3m 3s
build (macos-latest, default, automake autoconf libtool libsodium, libsodium, disabled)	3m 1s
	1h 3m 21s

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