[gdb/testsuite] Fix Wreturn-mismatch in gdb.base/list-dot-nodebug.exp

[gdb/testsuite] Fix Wreturn-mismatch in gdb.base/list-dot-nodebug.exp

When running test-case gdb.base/list-dot-nodebug.exp in a fedora rawhide
container, I run into:
...
temp/$pid/static-libc.c: In function 'main':
temp/$pid/static-libc.c:2:42: error: 'return' with a value, in function
 returning void [-Wreturn-mismatch]
    2 |                void main (void) { return 0; }
      |                                          ^
  ...
UNTESTED: gdb.base/list-dot-nodebug.exp: Can't statically link
...

Fix this by changing the return type to int.

Tested on x86_64-linux.