win-capture: Fix subprojects not installing on incremental builds

When changing a subproject (e.g. graphics-hook), the generated binaries
are not installed without also building win-capture. This commit enables
the subprojects to be treated as "plugins" which are installed to rundir
and also the installation directory without any additional workarounds.
This commit is contained in:
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f