gitmodules: Ignore 3rdparty in "diff family"

This should help avoid wrong 3rdparty commit ids
creeping in.

Change-Id: I2134ad1d3ad0237ef3f12baf4d4aafb02009e7bc
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/2768
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>
Tested-by: build bot (Jenkins)
1 file changed