BUILD: mjson: Fix warning about unused variables

BUILD: mjson: Fix warning about unused variables

clang 15 reports unused variables in src/mjson.c:

  src/mjson.c:196:21: fatal error: expected ';' at end of declaration
    int __maybe_unused n = 0;

and

  src/mjson.c:727:17: fatal error: variable 'n' set but not used [-Wunused-but-set-variable]
    int sign = 1, n = 0;

An issue was created on the project, but it was not fixed for now:

   https://github.com/cesanta/mjson/issues/51

So for now, to fix the build issue, these variables are declared as unused.
Of course, if there is any update on this library, be careful to review this
patch first to be sure it is always required.

This patch should fix the issue #1868. It be backported as far as 2.4.