Fix build with old 3.11 simgrid

src/common/thread.c

@@ -237,6 +237,9 @@ int starpu_pthread_key_delete(starpu_pthread_key_t key)
 
 /* We need it only when using smpi */
 #pragma weak smpi_process_get_user_data
+#if !HAVE_DECL_SMPI_PROCESS_SET_USER_DATA
+extern void *smpi_process_get_user_data();
+#endif
 
 int starpu_pthread_setspecific(starpu_pthread_key_t key, const void *pointer)
 {