sample_muxer: Silence MSVC warning.

Disable C4996; A warning that encourages users to make their code
require the "secure" variant of functions like tmpnam(). Doing so
just happens to make the resulting code require the MSVC runtime.

Change-Id: I0329ce3aa93da060110d24a4bc7f48a7f9c87227

sample_muxer.cpp

@@ -25,6 +25,10 @@
 
 using mkvmuxer::uint64;
 
+#ifdef _MSC_VER
+#pragma warning(disable:4996)
+#endif
+
 namespace {
 
 void Usage() {