s390: add missing header files to fix compilation

R=joransiu@ca.ibm.com

Change-Id: I26510b95469b6d9724bdae150f65f4c3068553ea
Reviewed-on: https://chromium-review.googlesource.com/c/1293774Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#56869}

src/s390/assembler-s390.h

@@ -51,6 +51,8 @@
 #include <vector>
 
 #include "src/assembler.h"
+#include "src/external-reference.h"
+#include "src/label.h"
 #include "src/s390/constants-s390.h"
 
 #define ABI_USES_FUNCTION_DESCRIPTORS 0