shader_ir: Remove unused includes

Removes unnecessary header dependencies.

src/video_core/shader/shader_ir.h

@@ -5,13 +5,10 @@
 #pragma once
 
 #include <array>
-#include <cstring>
 #include <map>
 #include <optional>
 #include <set>
-#include <string>
 #include <tuple>
-#include <variant>
 #include <vector>
 
 #include "common/common_types.h"