Fix build with current clang master

Compiling netd without the patch with current clang master errors out,
saying
cannot specify any part of a return type in the declaration of a conversion function; put the complete type after 'operator'

Change-Id: Ia49dd18f52982cf22b0aa8ade6b9a53ff24d26c0
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>
diff --git a/libnetdutils/include/netdutils/UniqueFd.h b/libnetdutils/include/netdutils/UniqueFd.h
index 3b63f34..61101f9 100644
--- a/libnetdutils/include/netdutils/UniqueFd.h
+++ b/libnetdutils/include/netdutils/UniqueFd.h
@@ -57,7 +57,7 @@
     void reset(Fd fd = Fd());
 
     // Implict cast to Fd
-    const operator Fd() const { return mFd; }
+    operator const Fd &() const { return mFd; }
 
   private:
     Fd mFd;