[bugfix] add missing headers

dune/common/math.hh

@@ -8,6 +8,7 @@
  */
 
 #include <cmath>
+#include <complex>
 
 namespace Dune
 {

dune/common/stringutility.hh

@@ -7,6 +7,10 @@
     \brief Miscellaneous helper stuff
  */
 
+#include <cstddef>
+#include <cstring>
+#include <algorithm>
+
 namespace Dune {
 
    /** @addtogroup Common