Properly link flecs library

engine/libs/flecs/test/custom_builds/c/rest/src/main.c

@@ -0,0 +1,9 @@
+#include <rest.h>
+
+int main(int argc, char *argv[]) {
+    ecs_world_t *world = ecs_init_w_args(argc, argv);
+
+    ecs_set(world, 0, EcsRest, {0});
+
+    return ecs_fini(world);
+}