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shadow uses non-literal format strings to integrate system information
into the login prompt. This is harmless because harmful format strings
are checked for (and in the worst case, the strings come from a file
that is writable by root only).

Removing -Werror=format-nonliteral here allows us to use
-Werror=format-nonliteral globally in OPTIMIZE_FOR=security mode
while keeping shadow building.

Signed-off-by: Bernhard Rosenkränzer <bernhard.rosenkraenzer.ext@huawei.com>