libstdc++: Move test error_category to global scope

libstdc++: Move test error_category to global scope

A recent GDB change causes this test to fail due to missing RTTI for the
custom_cast type. This is presumably because the custom_cat type was
defined as a local class, so has no linkage. Moving it to local scope
seems to fix the test regressions, and probably makes the test more
realistic as a local class with no linkage isn't practical to use as an
error category that almost certainly needs to be referred to in other
scopes.

libstdc++-v3/ChangeLog:

	* testsuite/libstdc++-prettyprinters/cxx11.cc: Move custom_cat
	to namespace scope.