libuci: fix false positive warning on older gcc versions

libuci: fix false positive warning on older gcc versions

Fixes the following warning:

  libuci.c: In function 'uci_set_conf2dir':
  libuci.c:97:59: error: argument 'dir' might be clobbered by 'longjmp' or 'vfork' [-Werror=clobbered]
     97 | int uci_set_conf2dir(struct uci_context *ctx, const char *dir)
        |                                               ~~~~~~~~~~~~^~~
  cc1: all warnings being treated as errors

Reported-by: Matthias Franck <[email protected]>
Signed-off-by: Felix Fietkau <[email protected]>