Fix swapped arguments for default build target list

[vjandrea]

The example plugins didn't build 'by default' without passing -DFFGL_BUILD_EXAMPLE_PLUGINS=ON explicitly.

When running cmake -S . -B build && cmake --build build --target help the ${FFGL_MASTER_PROJECT} toggle fell in the help text slot, and the docstring landed in the default argument, never evaluated as true, so the default plugins never appeared in the targets list.

With this change, the plugins show up :

cmake -S . -B build && cmake --build build --target help
-- The CXX compiler identification is GNU 13.3.0
...
... rebuild_cache
... ffgl-plugin-add
... ffgl-plugin-add-subtract
... ffgl-plugin-custom-thumbnails
... ffgl-plugin-events
... ffgl-plugin-gradients
... ffgl-plugin-particles
... ffgl-sdk
... source/lib/FFGLSDK.o
...