a64_get_set_elimination_pass: Make TrackingType enum an enum class

Prevents placing single letter enum members into the surrounding scope.

src/ir_opt/a64_get_set_elimination_pass.cpp

@@ -19,7 +19,7 @@ namespace Dynarmic::Optimization {
 void A64GetSetElimination(IR::Block& block) {
     using Iterator = IR::Block::iterator;
 
-    enum TrackingType {
+    enum class TrackingType {
         W, X,
         S, D, Q,
         SP, NZCV,