krb5_enomem to separate source file

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

lib/krb5/misc.c

@@ -86,13 +86,6 @@ out:
     return ret;
 }
 
-krb5_error_code
-krb5_enomem(krb5_context context)
-{
-    krb5_set_error_message(context, ENOMEM, N_("malloc: out of memory", ""));
-    return ENOMEM;
-}
-
 void
 _krb5_debug_backtrace(krb5_context context)
 {