Add settings for disk shader cache

src/core/settings.h

@@ -141,6 +141,7 @@ struct Values {
     bool use_gles;
     bool use_hw_renderer;
     bool use_hw_shader;
+    bool use_disk_shader_cache;
     bool shaders_accurate_mul;
     bool use_shader_jit;
     u16 resolution_factor;