Fix compilation error on non-Linux systems

[jgalenson]

The most recent commit (37dc6c6) breaks builds on non-Linux systems (or on Linux builds using Emscripten, which is how I hit it). Specifically, it adds a parameter to the Minisat::memUsedPeak function in utils/System.h but only modifies one version in utils/System.cc (under #if defined(linux)). The other copies of the function do not have the extra parameter and so no not compile.

The easiest fix is to simply modify the other three versions of this function to have the extra parameter, which they then ignore. A smarter implementation would of course use the parameter.

[Siesh1oo]

The unused parameter should also be void-casted to avoid the "unused parameter"-compiler-warning.