add missing includes detected in VS2015.

src/Operand.cpp

@@ -5,6 +5,7 @@
 #include "internal_includes/Shader.h"
 #include "internal_includes/HLSLCrossCompilerContext.h"
 #include "internal_includes/Instruction.h"
+#include <algorithm>
 
 uint32_t Operand::GetAccessMask() const
 {

src/toGLSLOperand.cpp

@@ -12,6 +12,7 @@
 
 #include <float.h>
 #include <stdlib.h>
+#include <algorithm>
 
 using namespace HLSLcc;
 

src/toMetalOperand.cpp

@@ -7,6 +7,7 @@
 #include "internal_includes/toMetal.h"
 #include <cmath>
 #include <sstream>
+#include <algorithm>
 
 #include <float.h>
 #include <stdlib.h>