Fixed build with NO_OPENAL defined

https://mantis.zdoom.org/view.php?id=596

src/sound/oalsound.cpp

@@ -122,6 +122,10 @@ void I_BuildALDeviceList(FOptionValues *opt)
 #endif
 }
 
+
+ReverbContainer *ForcedEnvironment;
+
+
 #ifndef NO_OPENAL
 
 
@@ -630,8 +634,6 @@ public:
 };
 
 
-ReverbContainer *ForcedEnvironment;
-
 #define AREA_SOUND_RADIUS  (32.f)
 
 #define PITCH_MULT (0.7937005f) /* Approx. 4 semitones lower; what Nash suggested */