Fix compiler warning

g++ warning:

    src/lstm/functions.h:152:35: warning:
        unused parameter ‘x’ [-Wunused-parameter]

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/lstm/functions.h

@@ -149,7 +149,7 @@ struct HPrime {
   }
 };
 struct UnityFunc {
-  inline double operator()(double x) const { return 1.0; }
+  inline double operator()(double /*x*/) const { return 1.0; }
 };
 struct IdentityFunc {
   inline double operator()(double x) const { return x; }