Fix LTO ODR build failure in wall.h

The conditional definition of a D2X flag causes a technical ODR
violation.  It is legal, though not useful, to define this flag in D1X
and in common code.  Remove the preprocessor guard so that D2X and
common code use the same definition for the enum.

common/main/wall.h

@@ -141,9 +141,9 @@ enum class wall_flag : uint8_t
 	door_auto = 1u << 4,	// Door automatically closes after time.
 	illusion_off = 1u << 5,	// Illusionary wall is shut off.
 	exploding = 1u << 6,
-#if defined(DXX_BUILD_DESCENT_II)
+	/* if DXX_BUILD_DESCENT_II */
 	buddy_proof = 1u << 7,	// Buddy assumes he cannot get through this wall.
-#endif
+	/* endif */
 };
 
 enum class wall_flags : uint8_t;