Properly link flecs library

engine/libs/flecs/examples/cpp/relationships/symmetric_relations/src/main.cpp

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+#include <symmetric_relations.h>
+#include <iostream>
+
+// Type for TradesWith relationship
+struct TradesWith { };
+
+int main(int, char*[]) {
+    flecs::world ecs;
+
+    // Register TradesWith as symmetric relationship. Symmetric relationships
+    // go both ways, adding (R, B) to A will also add (R, A) to B.
+    ecs.component<TradesWith>()
+        .add(flecs::Symmetric);
+
+    // Create two players
+    flecs::entity player_1 = ecs.entity();
+    flecs::entity player_2 = ecs.entity();
+
+    // Add (TradesWith, player_2) to player_1. This also adds 
+    // (TradesWith, player_1) to player_2.
+    player_1.add<TradesWith>(player_2);
+
+    // Log platoon of unit
+    std::cout << "Player 1 trades with Player 2: " << 
+        (player_1.has<TradesWith>(player_2) ? "true" : "false") << "\n";
+    std::cout << "Player 2 trades with Player 1: " << 
+        (player_2.has<TradesWith>(player_1) ? "true" : "false") << "\n";
+
+    // Output:
+    //  Player 1 trades with Player 2: true
+    //  Player 2 trades with Player 1: true
+}