fix: add forward declaration for ldst_inst_stats before tag_array

tag_array and baseline_cache use ldst_inst_stats* in their fill()
signatures, but the full class definition appears later in the header.
Add a forward declaration so the compiler recognizes the pointer type.

https://claude.ai/code/session_013NsDX12L3Cw97hFtCBFZrQ

Author: claude

src/gpgpu-sim/gpu-cache.h

@@ -953,6 +953,8 @@ class l2_cache_config : public cache_config {
   linear_to_raw_address_translation *m_address_mapping;
 };
 
+class ldst_inst_stats; // forward declaration
+
 class tag_array {
  public:
   // Use this constructor