[PATCH] Add const modifier to static functions in jsonb_utils.c

Hi,

It seems that not all parameters passed by pointer to static functions
defined in src/backend/utils/adt/jsonb_util.c are actually modified by
these functions. I propose a simple patch that adds const modifiers to
such parameters to make it clear that their values remain intact during
the call.

This modification was inspired by warnings produced by GCC 15.1 for
jsonb_util.c, discovered independently, but, as it turned out, already
described in this thread:
https://postgrespro.com/list/id/988bf1bc-3f1f-99f3-bf98-222f1cd9dc5e(at)xs4all(dot)nl
.

This is my first submission so I'm sorry in advance for any mistakes.
This modification was passed through pgindent, clearly compiles with GCC
15.1.1 on Arch Linux x86 64-bit, and ‘make check’ successfully passes
for it. I'm attaching the patch.

Best regards,
Dmitry