build: declare dependency for use as a subproject

This allows projects to override and build libplacebo by copying the repository to their ./subprojects/ directory.

To test, simply clone into the ./subprojects/ directory of any meson project which requires libplacebo. Since meson by default prefers to use the system libraries, even if a subprojects fallback exists, overriding is possible via:

meson setup --force-fallback-for libplacebo build