ncurses: remove -Werror=format-nonliteral from compiler flags

ncurses' termcap to terminfo converter uses a non-literal
format string.
This has been verified to be harmless in 6.2 (the "unsafe"
use of sscanf happens only with hardcoded, safe strings).

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

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