Make header files self-sufficient

Change-Id: Ice7d761b1023da77e50e5d6aa597964f7d9aa1d8
Reviewed-on: https://skia-review.googlesource.com/8302
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/src/core/SkStringUtils.h b/src/core/SkStringUtils.h
index fd158c3..fecf833 100644
--- a/src/core/SkStringUtils.h
+++ b/src/core/SkStringUtils.h
@@ -8,6 +8,8 @@
 #ifndef SkStringUtils_DEFINED
 #define SkStringUtils_DEFINED
 
+#include "SkScalar.h"
+
 class SkString;
 
 /**