krb5: fix compiler warning

Even though the variable was used in a DEBUGASSERT, GCC 8 warned in
debug mode:
krb5.c:324:17: error: unused variable 'maj' [-Werror=unused-variable]

Just suppress the warning and declare the variable unconditionally
instead of only for DEBUGBUILD (which also missed the check for
HAVE_ASSERT_H).

Closes https://github.com/curl/curl/pull/4020