I suspect you're getting the include file for std::stack rather than your Stack class. When I compiled this without using a separate header, I got no errors.
#include <file> generally means to look in the compiler's include directory first, while #include "file" generally means to look in the users directory first. Try changing the angle brackets to quotes.
If that doesn't work, change the name of the header file to something that doesn't conflict (e.g mystack.h).
p.s. When saying something doesn't compile, please post the exact error messages.