Add missing include for sleep

This fixes the AOSP mips build. Mirrors the same change needed in DM.cpp at crrev.com/1425843003

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Review URL: https://codereview.chromium.org/1568883002
diff --git a/bench/nanobench.cpp b/bench/nanobench.cpp
index 827ae03..5a83860 100644
--- a/bench/nanobench.cpp
+++ b/bench/nanobench.cpp
@@ -42,6 +42,10 @@
 
 #include <stdlib.h>
 
+#ifndef SK_BUILD_FOR_WIN32
+    #include <unistd.h>
+#endif
+
 #ifdef SK_BUILD_FOR_ANDROID_FRAMEWORK
     #include "nanobenchAndroid.h"
 #endif