gdb.base/nodebug.exp: Disable -Wdeprecated-non-prototype

Testing gdb.base/nodebug.exp with Clang shows:

 .../gdb.base/nodebug.c:63:1: warning: a function definition without a prototype is deprecated in all versions of C and is not supported in C23 [-Wdeprecated-non-prototype]
    63 | multf_noproto (v1, v2)
       | ^

Fix it by making gdb.base/nodebug.exp disable -Wdeprecated-non-prototype.

Tested with Clang 18.1.3 on x86_64-pc-linux-gnu, and Clang 21.0.0git
on x86_64-pc-windows-msvc.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: Id207053331377abec6918aa1188180d609c74273