Don't install library, .h, .pc and docs if used as a subproject
Since this library is theoretically API-unstable, eos-payg references it
as a git submodule in subprojects/ so it can be accessed via Meson's
subproject() function. However, ninja install
for eos-payg also
installs lots of pieces of libgsystemservice in this case.
To avoid this, teach libgsystemservice to not install anything when used as a subproject.