libstdc++: Fix typo in std::stacktrace::max_size [PR115063]

libstdc++-v3/ChangeLog:

	PR libstdc++/115063
	* include/std/stacktrace (basic_stacktrace::max_size): Fix typo
	in reference to _M_alloc member.
	* testsuite/19_diagnostics/stacktrace/stacktrace.cc: Check
	max_size() compiles.