[gdb/testsuite] Fix gdb.base/ptype.exp with gcc 15

[gdb/testsuite] Fix gdb.base/ptype.exp with gcc 15

With test-case gdb.base/ptype.exp and gcc 15 I run into:
...
(gdb) ptype old_fptr^M
type = double (*)(void)^M
(gdb) FAIL: $exp: ptype old_fptr (compiler doesn't emit unprototyped types)
...

Since C23, non-prototype function declarations are no longer supported, so
"double (*old_fptr) ()" is interpreted as "double (*old_fptr) (void)".

We could try to fix this by detecting the language dialect used, and accepting
the output in that case, but that feels fragile.

We could try to fix this by hard-coding the language dialect, but that doesn't
work for all compilers.

So instead, we opt for the simplest solution: just accept this output, and
produce a pass.

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR testsuite/32756
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32756