Fix building with gcc-4.6

https://bugs.gentoo.org/show_bug.cgi?id=363467

Patch written by Kacper Kowalik <xarthisius@gentoo.org>
--- a/src/Graphics/GRA_thiessenTriangulation.h
+++ b/src/Graphics/GRA_thiessenTriangulation.h
@@ -19,6 +19,7 @@
 #define GRA_THIESSENTRIANGULATION
 
 #include <vector>
+#include <cstddef>
 
 class GRA_thiessenTriangulation
 {