Fix CMake-generated `libzmq.pc` file #1203

Job	Run time
build (x64, release, windows-2019, ON, ON, Visual Studio 16 2019, v142, vs2019, v142-x64, ON)	5m 18s
cron	0s
build (ubuntu-latest, default, asciidoctor, disabled, select)	2m 39s
build (ubuntu-latest, default, disabled, poll)	2m 25s
build (ubuntu-latest, android, disabled)	3m 20s
build (ubuntu-latest, coverage, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev lcov, enabled,...	3m 11s
build (ubuntu-latest, valgrind, valgrind libgnutls28-dev, enabled)	8m 22s
build (ubuntu-latest, cmake, libsodium, enabled, cmake libsodium-dev, enabled)	3m 24s
build (ubuntu-latest, cmake, libsodium, enabled, enabled, cmake libsodium-dev libkrb5-dev, enabled)	3m 24s
build (ubuntu-latest, cmake, enabled, cmake clang-format-11, 1)	35s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 24s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	3m 11s
build (ubuntu-latest, default, libkrb5-dev libnorm-dev libpgm-dev libgnutls28-dev libsodium-dev l...	2m 51s
build (ubuntu-latest, abi-compliance-checker, abi-dumper abi-compliance-checker, disabled)	3m 12s
build (ubuntu-latest, cmake, clang-tidy clang-tools, enabled, clang++)	3m 29s
build (macos-latest, default, automake autoconf libtool, enabled)	3m 13s
build (macos-latest, default, automake autoconf libtool libsodium, libsodium, disabled)	1m 26s
	53m 24s

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