Replace deprecated get_pkgconfig_variable with get_variable

- Meson 0.57 warns that get_pkgconfig_variable is deprecated and
  we should be using get_variable instead.

Tested By:

Setup docker, and made sure the service files are installed in the
desired directory.

Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/pldmd.service to /lib/systemd/system
Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/pldm_verbosity to /etc/default
Installing /home/juliet/Documents/Work/docker/ci_test_area/pldm/build/softoff/pldmSoftPowerOff.service to /lib/systemd/system

Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>
Change-Id: I2da6f5fecc37e91eb927b2ebae9bd59de59d1b08