Warnings: Fixed a few missing-return warnings in video_core.

src/video_core/swrasterizer/framebuffer.cpp

@@ -352,6 +352,8 @@ u8 LogicOp(u8 src, u8 dest, FramebufferRegs::LogicOp op) {
     case FramebufferRegs::LogicOp::OrInverted:
         return ~src | dest;
     }
+
+    UNREACHABLE();
 };
 
 } // namespace Rasterizer