Fix google-explicit-constructor warnings.

Bug: 28341362
Change-Id: I420d23cc1f7ed44e1d9a215ee6815afab365046e
diff --git a/mmap-perf/mmapPerf.cpp b/mmap-perf/mmapPerf.cpp
index 16e0c76..bbc83c7 100644
--- a/mmap-perf/mmapPerf.cpp
+++ b/mmap-perf/mmapPerf.cpp
@@ -20,7 +20,7 @@
     int get() { return m_fd; }
     void set(int fd) { m_fd = fd; }
     Fd() {}
-    Fd(int fd) : m_fd{fd} {}
+    explicit Fd(int fd) : m_fd{fd} {}
     ~Fd() {
         if (m_fd >= 0)
             close(m_fd);