Make COROUTINE_XMM_REGISTERS compile-time only too

coroutine/win64/Context.h

@@ -16,8 +16,10 @@ extern "C" {
 
 #define COROUTINE __declspec(noreturn) void
 
-enum {COROUTINE_REGISTERS = 8};
-const size_t COROUTINE_XMM_REGISTERS = 1+10*2;
+enum {
+    COROUTINE_REGISTERS = 8,
+    COROUTINE_XMM_REGISTERS = 1+10*2,
+};
 
 typedef struct
 {