Add a missing include.

Was being included transitively by stlport.

Bug: 15193147
Change-Id: If462e5ddcf4a620367fb4b5533f44960fd3b7378
diff --git a/libs/gui/tests/SRGB_test.cpp b/libs/gui/tests/SRGB_test.cpp
index 2d5b8aa..da2add7 100644
--- a/libs/gui/tests/SRGB_test.cpp
+++ b/libs/gui/tests/SRGB_test.cpp
@@ -19,6 +19,8 @@
 
 #include "GLTest.h"
 
+#include <math.h>
+
 #include <gui/CpuConsumer.h>
 #include <gui/Surface.h>
 #include <gui/SurfaceComposerClient.h>