crypto: make krb5_enomem a static inline function

With the recent update to the imported heimdal code, krb5_enomem
is used in a few places as a simple statement that doesn't make
use of the value.  With the current definition, this triggers
compiler warnings because the statement has no effect.

Replace the definition with a static inline function that returns
the expected value.

Change-Id: I799022b69a43ab635560a1a5ea8f67f0a8f7230a
Reviewed-on: http://gerrit.openafs.org/9335
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Simon Wilkinson <simonxwilkinson@gmail.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>