Add O_CLOEXEC flag when opening file

This fixes a buid warning in aosp/2163081.

Test: atest MicrodroidBenchmarks
Change-Id: I52128a56c9ed3427d029828e9a63ec695192131a

tests/benchmark/src/native/benchmarkbinary.cpp

@@ -68,7 +68,7 @@ private:
         char buf[kBlockSizeBytes];
 
         clock_t start = clock();
-        unique_fd fd(open(filename.c_str(), O_RDONLY));
+        unique_fd fd(open(filename.c_str(), O_RDONLY | O_CLOEXEC));
         if (fd.get() == -1) {
             return ErrnoError() << "Read: opening " << filename << " failed";
         }