Add a dependency XLNX_ZYNQMP -> PTIMER to fix:

  /usr/bin/ld:
  libcommon.fa.p/hw_net_can_xlnx-zynqmp-can.c.o: in function `xlnx_zynqmp_can_realize':
  hw/net/can/xlnx-zynqmp-can.c:1082: undefined reference to `ptimer_init'
  hw/net/can/xlnx-zynqmp-can.c:1085: undefined reference to `ptimer_transaction_begin'
  hw/net/can/xlnx-zynqmp-can.c:1087: undefined reference to `ptimer_set_freq'
  hw/net/can/xlnx-zynqmp-can.c:1088: undefined reference to `ptimer_set_limit'
  hw/net/can/xlnx-zynqmp-can.c:1089: undefined reference to `ptimer_run'
  hw/net/can/xlnx-zynqmp-can.c:1090: undefined reference to `ptimer_transaction_commit'
  libcommon.fa.p/hw_net_can_xlnx-zynqmp-can.c.o:(.data.rel+0x2c8): undefined reference to `vmstate_ptimer'

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210131184449.382425-6-f4bug@amsat.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>