Add use Carp; to file to prevent the program from dying when "croak" …

…is called.

Minor fix.  There is an underlying issue, or possible issue, where there is a warning because
of a conflict between a login name and a cookie, possibly left over or stale.  Quitting the browser
fixes this. Is this a bug or a feature?  I suspect it's new behavior with the added LTI authentication.

lib/WeBWorK/Authen.pm

@@ -57,6 +57,7 @@ use WeBWorK::Debug;
 use WeBWorK::Utils qw/writeCourseLog runtime_use/;
 use WeBWorK::Localize;
 use URI::Escape;
+use Carp;
 
 use mod_perl;
 use constant MP2 => ( exists $ENV{MOD_PERL_API_VERSION} and $ENV{MOD_PERL_API_VERSION} >= 2 );