GCC 4.5 compatibility: missing includes.

src/DimensionTraits.h

@@ -15,6 +15,7 @@
     GNU Lesser General Public License version 3 for more details.
 */
 
+#include <cstddef>
 #include <cstdint>
 
 /** @file

src/Magnum.h

@@ -19,6 +19,7 @@
  * @brief Basic definitions
  */
 
+#include "magnumCompatibility.h"
 #include "magnumConfigure.h"
 
 #ifndef MAGNUM_TARGET_GLES

src/Math/Math.h

@@ -16,6 +16,7 @@
 */
 
 #include <cmath>
+#include <cstdint>
 #include <type_traits>
 #include <limits>
 #include <algorithm>

src/Math/MathTypeTraits.h

@@ -19,6 +19,7 @@
  * @brief Class Magnum::Math::MathTypeTraits
  */
 
+#include <cstddef>
 #include <cstdint>
 #include <cmath>