[fieldvector] add missing include of ftraits.hh

(cherry picked from commit 7135cbb0)
Signed-off-by: Carsten Gräser <graeser@dune-project.org>

dune/common/fvector.hh

@@ -17,6 +17,8 @@
 #include "typetraits.hh"
 #include "exceptions.hh"
 #include "array.hh"
+
+#include "ftraits.hh"
 #include "densevector.hh"
 #include "unused.hh"