libretro: Minor mutex fix

[RobLoach]

• I confirm that I am the author of this code and release it to the PhysicsFS project under the Zlib license. This contribution does not contain code from other sources, including code generated by a Large Language Model ("AI").

This is a minor change to return a 1 on grabMutex when on a single-threaded application. This mimics the same behavior across other platforms, like playdate.