basic/user-util: properly protect use of gshadow

Commit 100d5f6e (user-util: add new wrappers for [...] database
files), ammended by commit 4f07ffa8 (Use #if instead of #ifdef for
ENABLE_GSHADOW) moved code from sysuser to basic/user-util.

In doing so, the combination of both commits properly propagated the
ENABLE_GSHADOW conditions around the function manipulating gshadow, but
they forgot to protect the inclusion of the gshadow.h header.

Fix that to be able to build on C libraries that do not provide gshadow
(e.g. uClibc-ng, where it does not exist.)