devel/sdbus-cpp: unbreak pkg-config support

$ pkg-config --libs sdbus-c++
Package libsystemd was not found in the pkg-config search path.
Perhaps you should add the directory containing `libsystemd.pc'
to the PKG_CONFIG_PATH environment variable
Package 'libsystemd', required by 'sdbus-c++', not found

(cherry picked from commit 53705c8b604a4ccc55220a9b742aa5a9324c4124)