IWYU: 'core' target, files starting A-C.

TBR=reed@google.com
Verbal lgtm, does not change API.

Committed: https://skia.googlesource.com/skia/+/7403d87db8e43d4c2b5b25ac22a0ebc22bd09d69

Review URL: https://codereview.chromium.org/1265033002
diff --git a/gm/hairlines.cpp b/gm/hairlines.cpp
index 6056415..9d7e366 100644
--- a/gm/hairlines.cpp
+++ b/gm/hairlines.cpp
@@ -7,6 +7,7 @@
 
 #include "gm.h"
 #include "SkCanvas.h"
+#include "SkPath.h"
 #include "SkTArray.h"
 
 namespace skiagm {