Prevent compilation issues with boost-1.60.0.

Reported upstream at https://github.com/csound/csound/issues/570

--- Csound6.05/Opcodes/chua/ChuaOscillator.cpp	2015-04-25 14:06:22.995646234 -0500
+++ Csound6.05/Opcodes/chua/ChuaOscillator.cpp	2015-12-21 10:31:58.182362640 -0600
@@ -117,11 +117,12 @@
 //     d = sys_variables(12);
 //     gnor = a*(x.^3) + b*(x.^2) + c*x + d;
 
-#include <OpcodeBase.hpp>
 #include <boost/numeric/ublas/vector.hpp>
 using namespace boost::numeric;
 #include <cmath>
 
+#include <OpcodeBase.hpp>
+
 #undef CS_KSMPS
 #define CS_KSMPS     (opds.insdshead->ksmps)