Reverse dependency between SkScalar.h and SkFixed.h.

The following are unused in Chromium, Android, Mozilla, and Google3:
 - SkFixedToScalar
 - SkScalarToFixed

The following are additionally unused in Skia:
 - SkStrAppendFixed
 - SkWriteBuffer::writeFixed

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1841123002

Review URL: https://codereview.chromium.org/1841123002
diff --git a/tests/MathTest.cpp b/tests/MathTest.cpp
index 5bc2d0e..580cd94 100644
--- a/tests/MathTest.cpp
+++ b/tests/MathTest.cpp
@@ -7,6 +7,7 @@
 
 #include "SkColorPriv.h"
 #include "SkEndian.h"
+#include "SkFixed.h"
 #include "SkFloatBits.h"
 #include "SkFloatingPoint.h"
 #include "SkHalf.h"