Fix failing build due to missing include

Fix the build of MixHelpers.cpp which fails due to a missing include of
cstdio (printf is not defined).

src/core/MixHelpers.cpp

@@ -26,6 +26,8 @@
 #include "lmms_math.h"
 #include "ValueBuffer.h"
 
+#include <cstdio>
+
 
 static bool s_NaNHandler;