Enable systemd_sink tests in linux pipeline

[MyNameIsLame]

As mentioned in #2668, the test_systemd.cpp tests are not build and executed in the linux pipeline, as pkg-config and libsystemd-dev packages are not installed.

In tests/CMakeLists.txt:

find_package(PkgConfig)
if(PkgConfig_FOUND)
    pkg_check_modules(systemd libsystemd)
endif()

...

if(systemd_FOUND)
    list(APPEND SPDLOG_UTESTS_SOURCES test_systemd.cpp)
endif()

By installing the two packages, the test_systemd.cpp tests should be built.

Note: Without #2668 the tests will not compile.

[gabime]

Thanks @MyNameIsLame