Fix GCC warning about unused value

Casting to void should shut up GCC about

error: statement has no effect [-Werror=unused-value]

static_cast<void>() is possible for all current types. Supposedly an old
C style cast could also be used, but they should not be used in modern
C++ code.

src/tabcontrollers/audiomanager/AudioManagerDummy.cpp

@@ -1,7 +1,8 @@
 #include "AudioManagerDummy.h"
 
 // Used to get the compiler to shut up about C4100: unreferenced formal parameter.
-#define UNREFERENCED_PARAMETER(P) (P)
+// The cast is to get GCC to shut up about it.
+#define UNREFERENCED_PARAMETER(P) static_cast<void>((P))
 
 // application namespace
 namespace advsettings {

src/tabcontrollers/keyboardinput/KeyboardInputDummy.cpp

@@ -1,7 +1,8 @@
 #include "KeyboardInputDummy.h"
 
 // Used to get the compiler to shut up about C4100: unreferenced formal parameter.
-#define UNREFERENCED_PARAMETER(P) (P)
+// The cast is to get GCC to shut up about it.
+#define UNREFERENCED_PARAMETER(P) static_cast<void>((P))
 
 namespace advsettings {