add missing 'extern "C"' before including quadmath.h

again, that could be considered a bug in quadmath.h

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@6535 2fb0f335-1f38-0410-981e-8018bf24f1b0

dumux/common/quad.hh

@@ -30,7 +30,10 @@
 
 #include <iostream>
 #include <cmath>
+
+extern "C" {
 #include <quadmath.h>
+}
 
 typedef __float128 quad;