krb5_enomem() is a wrapper around krb5_set_error_message() which
is used throughout the lib/krb5 sources. Some of the lib/krb5
sources are imported into third party projects and those projects
must be able to pull in krb5_enomem() without other baggage.
Create a new source file lib/krb5/enomem.c.
Change-Id: Id109386d48e3e2988b705b82525adf4f1fa5ea98
