hwui: add missing <mutex> include

Upgrading to clang-r522817's new libc++ requires that <mutex> be
included here.

Bug: 333165689
Test: treehugger
Flag: EXEMPT, fix build error
Change-Id: I14ff396b21feb4b569a3c8d967c8295a04005beb
diff --git a/libs/hwui/pipeline/skia/ShaderCache.cpp b/libs/hwui/pipeline/skia/ShaderCache.cpp
index b870023..8e07a2f 100644
--- a/libs/hwui/pipeline/skia/ShaderCache.cpp
+++ b/libs/hwui/pipeline/skia/ShaderCache.cpp
@@ -15,14 +15,18 @@
  */
 
 #include "ShaderCache.h"
+
 #include <GrDirectContext.h>
 #include <SkData.h>
 #include <gui/TraceUtils.h>
 #include <log/log.h>
 #include <openssl/sha.h>
+
 #include <algorithm>
 #include <array>
+#include <mutex>
 #include <thread>
+
 #include "FileBlobCache.h"
 #include "Properties.h"