Make ScopedFD's bool operator explicit

This prevents an implicit conversion from bool->int to ensure the user is
calling .get() properly.

Signed-off-by: Maxwell Henderson <mxwhenderson@gmail.com>
Change-Id: I914c0e9583fbfcaa7fafeb0474dbe068716ae19d
diff --git a/aos/scoped/scoped_fd.h b/aos/scoped/scoped_fd.h
index 116985a..e393e89 100644
--- a/aos/scoped/scoped_fd.h
+++ b/aos/scoped/scoped_fd.h
@@ -35,7 +35,7 @@
       fd_ = new_fd;
     }
   }
-  operator bool() const { return fd_ != -1; }
+  explicit operator bool() const { return fd_ != -1; }
 
  private:
   int fd_;