zip: Remove -Werror=format-nonliteral from compiler flags

zip uses some #define trickery that triggers warnings with
-Wformat-nonliteral.
This has been verified to be harmless in 3.0.

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

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