[RFA/doco 1/4] gdb/NEWS: Document that building GDB now requires GMP

[Eli Zaretskii]

> From: Joel Brobecker <brobecker@adacore.com>
> Date: Sun, 22 Nov 2020 15:56:27 +0400
> Cc: Joel Brobecker <brobecker@adacore.com>
> 
> gdb/ChangeLog:
> 
>         * NEWS: Document that building GDB now requires GMP.

OK.