Move SkOpts.h back to src/core.

The Chrome opts targets (sse2, ssse3, sse41, etc) don't have include/private on
their include path.  This should unblock the roll.

TBR=reed@google.com

BUG=skia:4117

Review URL: https://codereview.chromium.org/1268853007

include/core/SkUtils.h

@@ -9,7 +9,11 @@
 #define SkUtils_DEFINED
 
 #include "SkTypes.h"
-#include "../private/SkOpts.h"
+
+namespace SkOpts {
+    extern void (*memset16)(uint16_t[], uint16_t, int);
+    extern void (*memset32)(uint32_t[], uint32_t, int);
+}
 
 ///////////////////////////////////////////////////////////////////////////////