gmp: Remove -Werror=format-nonliteral to compiler flags

gmp uses non-literal format strings to implement
its own printing functions.
This has been verified to be harmless in 6.2.0.

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

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