fixed linux build for relation completer

include/matador/utils/primary_key_attribute.hpp

@@ -1,11 +1,13 @@
 #ifndef PRIMARY_KEY_ATTRIBUTE_HPP
 #define PRIMARY_KEY_ATTRIBUTE_HPP
 
+#include <cstddef>
+
 namespace matador::utils {
 class primary_key_attribute final {
 public:
     /**
-     * Creates primary_key_attribute instance
+     * Creates a primary_key_attribute instance
      * with size 0 (zero).
      */
     primary_key_attribute() = default;