test: add missing include

test/common/aggregate.h

@@ -1,6 +1,7 @@
 #ifndef ENTT_COMMON_AGGREGATE_H
 #define ENTT_COMMON_AGGREGATE_H
 
+#include <compare>
 #include <type_traits>
 
 namespace test {

test/common/pointer_stable.h

@@ -1,6 +1,8 @@
 #ifndef ENTT_COMMON_POINTER_STABLE_H
 #define ENTT_COMMON_POINTER_STABLE_H
 
+#include <compare>
+
 namespace test {
 
 struct pointer_stable {