Move math.h include out of viewport_type.h, fixes gcc 6 compilation.

Make a system include.

src/viewport.cpp

@@ -86,6 +86,7 @@
 #include "bridge_map.h"
 
 #include <map>
+#include <math.h>
 
 #include "table/strings.h"
 #include "table/string_colours.h"

src/viewport_type.h

@@ -15,7 +15,6 @@
 #include "zoom_type.h"
 #include "strings_type.h"
 #include "table/strings.h"
-#include "math.h"
 
 class LinkGraphOverlay;