Provide template gai.conf in glibc-doc
The gai.conf template used to be shipped via glibc-common, but was
incorrectly removed during 1fa03e97ae4cb4 ("Introduce new
glibc-doc.noarch subpackage (#1346925)" as unnecessary. However, it has
value as a template file and shipping it is preferable. This commit
reinstates the file at /usr/share/doc/glibc/gai.conf as part of the
glibc-doc subpackage.