Add missing includes to fix build

Change-Id: Ifee7a5b4591e640fb99642ff3ddb0e7dd23425ae
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/45381
Commit-Queue: Ben Clayton <bclayton@chromium.org>
Auto-Submit: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/reader/spirv/entry_point_info.h

@@ -17,6 +17,8 @@
 
 #include <string>
 
+#include "src/ast/pipeline_stage.h"
+
 namespace tint {
 namespace reader {
 namespace spirv {

src/semantic/type_mappings.h

@@ -15,6 +15,8 @@
 #ifndef SRC_SEMANTIC_TYPE_MAPPINGS_H_
 #define SRC_SEMANTIC_TYPE_MAPPINGS_H_
 
+#include <type_traits>
+
 namespace tint {
 
 // Forward declarations