Only restore parent context's \@ value if it had one defined (#1366)

This way, if a child context initializes `\@`, the parent won't
reset it. And if the child context did not initialize `\@`,
then resetting it would be redundant.

include/asm/fstack.hpp

@@ -14,8 +14,6 @@
 
 #include "linkdefs.hpp"
 
-#include "asm/lexer.hpp"
-
 struct FileStackNode {
 	FileStackNodeType type;
 	std::variant<