tests: fix linking test-char on win32

test.char.exe fails to link:
qemu-char.o: In function `win_chr_free':
/home/elmarco/src/qemu/qemu-char.c:2149: undefined reference to `qemu_del_polling_cb'
/home/elmarco/src/qemu/qemu-char.c:2151: undefined reference to `qemu_del_polling_cb'
qemu-char.o: In function `win_stdio_thread':
/home/elmarco/src/qemu/qemu-char.c:2568: undefined reference to `qemu_del_wait_object'
qemu-char.o: In function `qemu_chr_open_stdio':
/home/elmarco/src/qemu/qemu-char.c:2661: undefined reference to `qemu_add_wait_object'
/home/elmarco/src/qemu/qemu-char.c:2646: undefined reference to
`qemu_add_wait_object'
...

It needs main-loop.o symbols, among others. Linking with
$(test-block-obj-y) brings what's necessary. We could try to eventually
strip to the minimum if needed.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
