systemd: Correctly check ENABLE_GSHADOW with if/endif

systemd: Correctly check ENABLE_GSHADOW with if/endif

Use commit 4f07ffa8f5ab ("Use #if instead of #ifdef for ENABLE_GSHADOW")
from upstream to check ENABLE_GSHADOW correctly that is defined as 0
in case of musl.

While there replace specific patch with one from upstream that does
exactly the same commit 66a5b5ce9b99 ("basic/user-util: properly protect
use of gshadow").

Signed-off-by: Serhey Popovych <serhe.popovych@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>