kernel: stack: Making if's body a compound statement

MISRA-C rule 15.6

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>
diff --git a/kernel/stack.c b/kernel/stack.c
index 7924b8f..4a22693 100644
--- a/kernel/stack.c
+++ b/kernel/stack.c
@@ -155,8 +155,9 @@
 	}
 
 	result = _pend_current_thread(key, &stack->wait_q, timeout);
-	if (result == -EAGAIN)
+	if (result == -EAGAIN) {
 		return -EAGAIN;
+	}
 
 	*data = (u32_t)_current->base.swap_data;
 	return 0;