MM Best Fit Plane - Fix missing include on GCC/Linux

The code is failing to build on Linux with GCC, because it's missing the
"cstring" header, which the spec says it should use, but MSVC doesn't
care about.

https://en.cppreference.com/w/cpp/string/byte/strcmp

src/mmSolver/cmd/MMBestFitPlaneCmd.cpp

@@ -75,6 +75,7 @@
 
 // STL
 #include <cassert>
+#include <cstring>
 
 // Maya
 #include <maya/MArgDatabase.h>