services: configuration: Fix %location accessor name.

services: configuration: Fix %location accessor name.

The non-hygienic binding of the source location accessor was set to
'-location'; in modules where multiple configurations were defined, it would
cause compilation warnings such as:

    gnu/services/mail.scm:175:0: warning: shadows previous definition of
    `%-location-procedure' at gnu/services/mail.scm:165:0

* gnu/services/configuration.scm (define-configuration-helper) <id>: Add a
missing #'stem argument to use the configuration name as a prefix to the
location accessor identifier.