video_core: unbreak -Werror in NVDEC with Clang

src/core/hle/service/nvdrv/devices/nvhost_nvdec_common.cpp:41:15: error: unused variable 'OutOfMemory' [-Werror,-Wunused-const-variable]
constexpr u32 OutOfMemory{static_cast<u32>(-12)};
              ^