Fix FTBFS with gcc6

--- src/ffadotypes.h	2010-01-01 23:04:37.000000000 +0100
+++ src/ffadotypes.h	2016-10-03 18:14:14.161886793 +0200
@@ -51,6 +51,8 @@
 };
 
 
+#include <cmath>
+#include <algorithm>
 #include <vector>
 #include <string>