header self-sufficiency and idempotency

Change-Id: I4ec74a358e11d85c6eaf1658efd6f1391ee5e387
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223930
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
diff --git a/src/image/SkReadPixelsRec.h b/src/image/SkReadPixelsRec.h
index 2257e5c..dd88d6c 100644
--- a/src/image/SkReadPixelsRec.h
+++ b/src/image/SkReadPixelsRec.h
@@ -9,6 +9,7 @@
 #define SkReadPixelsRec_DEFINED
 
 #include "include/core/SkImageInfo.h"
+#include "include/core/SkPixmap.h"
 
 /**
  *  Helper class to package and trim the parameters passed to readPixels()