Add missing include

[davide125]

This fixes the following build issue with GCC 15

/builddir/build/BUILD/psmoveapi-4.0.12_20240424git26e1446-build/psmoveapi-26e1446fc9b5996192a1f406d289e64e4e97a195/examples/c/test_navcon.cpp:48:13: error: ‘strcmp’ was not declared in this scope
   48 |         if (strcmp(name, "Sony Navigation Controller") == 0) {
      |             ^~~~~~
/builddir/build/BUILD/psmoveapi-4.0.12_20240424git26e1446-build/psmoveapi-26e1446fc9b5996192a1f406d289e64e4e97a195/examples/c/test_navcon.cpp:40:1: note: ‘strcmp’ is defined in header ‘<cstring>’; this is probably fixable by adding ‘#include <cstring>’
   39 | #include "psmove.h"
  +++ |+#include <cstring>
   40 |