Makefile: Only enable I2C programmers on Linux

The I2C support code they use is Linux-specific.

Change-Id: I04d25ff8f8c3097428ac8695669b1757c38f49e9
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/53999
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
1 file changed