[gdb/testsuite] Fix gdb.base/bp-permanent.exp with gcc 15

[gdb/testsuite] Fix gdb.base/bp-permanent.exp with gcc 15

With test-case gdb.base/bp-permanent.exp and gcc 15 I run into:
...
gdb compile failed, bp-permanent.c: In function 'test_signal_nested':
bp-permanent.c:118:20: error: passing argument 2 of 'signal' from \
  incompatible pointer type [-Wincompatible-pointer-types]
  118 |   signal (SIGALRM, test_signal_nested_handler);
      |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~
      |                    |
      |                    void (*)(void)
In file included from bp-permanent.c:20:
/usr/include/signal.h:88:57: note: expected '__sighandler_t' \
  {aka 'void (*)(int)'} but argument is of type 'void (*)(void)'
...

Fix this by adding an int parameter to test_signal_nested_handler.

Tested on x86_64-linux.

PR testsuite/32756
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32756