[gdb/testsuite] Fix gdb.dwarf2/dw2-unspecified-type-foo.c with -m32

When running test-case gdb.dwarf2/dw2-unspecified-type-foo.c with target board
unix/-m32, I run into:
...
(gdb) PASS: gdb.dwarf2/dw2-unspecified-type.exp: ptype foo
p ((int (*) ()) foo) ()^M
$1 = -135698472^M
...

Add the missing "return 0" in foo, which fixes this.

Tested on x86_64-linux.