gl_shader_disk_cache: Make variable non-const in decompiled entry case

std::move does nothing when applied to a const variable. Resources can't
be moved if the object is immutable. With this change, we don't end up
making several unnecessary heap allocations and copies.

src/video_core/renderer_opengl/gl_shader_disk_cache.cpp

@@ -243,7 +243,7 @@ ShaderDiskCacheOpenGL::LoadPrecompiledFile(FileUtil::IOFile& file) {
                 return {};
             }
 
-            const auto entry = LoadDecompiledEntry();
+            auto entry = LoadDecompiledEntry();
             if (!entry) {
                 return {};
             }