demos/settings: move parse_args out of NUKLEAR ifdef

In case demos are compiled, but the nuklear headers are not available,
compilation fails, because the implementation of parse_args function
is protected by the HAVE_NUKLEAR preprocessor macro.

To avoids this issue, move this implementation of this function outside
of this ifdef.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>