fpga: Protect GZIP usage when LOADMK is enabled

For case where CMD_FPGA_LOADMK is enabled and GZIP disable.

Warning log:
common/built-in.o: In function `do_fpga':
/mnt/disk/u-boot/common/cmd_fpga.c:218: undefined reference to `gunzip'

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>