Missing includes.

YES! The last commit before making MSVC 2015 green and I broke build
*everywhere*. Dammit.

src/Magnum/Implementation/QueryState.cpp

@@ -25,6 +25,7 @@
 
 #include "QueryState.h"
 
+#include "Magnum/AbstractQuery.h"
 #include "Magnum/Context.h"
 #include "Magnum/Extensions.h"
 

src/Magnum/Implementation/QueryState.h

@@ -25,6 +25,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include <string>
 #include <vector>
 
 #include "Magnum/Magnum.h"

src/Magnum/Implementation/TransformFeedbackState.cpp

@@ -1,5 +1,6 @@
 #include "TransformFeedbackState.h"
 
+#include "Magnum/Context.h"
 #include "Magnum/Extensions.h"
 #include "Magnum/TransformFeedback.h"
 

src/Magnum/Implementation/TransformFeedbackState.h

@@ -25,6 +25,7 @@
     DEALINGS IN THE SOFTWARE.
 */
 
+#include <string>
 #include <vector>
 
 #include "Magnum/Magnum.h"