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.

libstdc++-v3/testsuite/libstdc++-prettyprinters/cxx11.cc

@@ -63,6 +63,11 @@ struct datum
 
 std::unique_ptr<datum> global;
 
+struct custom_cat : std::error_category {
+  const char* name() const noexcept { return "miaow"; }
+  std::string message(int) const { return ""; }
+};
+
 int
 main()
 {
@@ -179,10 +184,7 @@ main()
   std::error_condition ecinval = std::make_error_condition(std::errc::invalid_argument);
   // { dg-final { note-test ecinval {std::error_condition = {"generic": EINVAL}} } }
 
-  struct custom_cat : std::error_category {
-    const char* name() const noexcept { return "miaow"; }
-    std::string message(int) const { return ""; }
-  } cat;
+  custom_cat cat;
   std::error_code emiaow(42, cat);
   // { dg-final { note-test emiaow {std::error_code = {custom_cat: 42}} } }
   std::error_condition ecmiaow(42, cat);