CMakeLists.txt: remove -Werror=format-nonliteral

[hauke]

The musl libc fortify source headers are using a nonliteral format string in the sprintf() and snprintf() function. Do not fail in ucode

Not everyone is using the workaround in fortify source headers.

[jow-]

FIrst I would like to better understand the actual error this is fixing

[hauke]

We are seeing such errors when activatig fortify sources for cmake applications:

/include/fortify/stdio.h: In function 'snprintf':
/include/fortify/stdio.h:101:9: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  101 |         return __orig_snprintf(__s, __n, __f, __builtin_va_arg_pack());
      |         ^~~~~~
/include/fortify/stdio.h: In function 'sprintf':
/include/fortify/stdio.h:110:17: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  110 |                 __r = __orig_snprintf(__s, __b, __f, __builtin_va_arg_pack());
      |                 ^~~
/include/fortify/stdio.h:114:17: error: format not a string literal, argument types not checked [-Werror=format-nonliteral]
  114 |                 __r = __orig_sprintf(__s, __f, __builtin_va_arg_pack());
      |                 ^~~
cc1: all warnings being treated as errors
ninja: build stopped: subcommand failed.

I would like to move away from the workaround in OpenWrt: openwrt/openwrt@b237266