nvmap: Add a return 0 underneath the UNIMPLEMENTED macro

This macro resolves to an empty macro in release builds.

src/core/hle/service/nvdrv/devices/nvmap.cpp

@@ -36,6 +36,7 @@ u32 nvmap::ioctl(u32 command, const std::vector<u8>& input, std::vector<u8>& out
     }
 
     UNIMPLEMENTED();
+    return 0;
 }
 
 u32 nvmap::IocCreate(const std::vector<u8>& input, std::vector<u8>& output) {