build-unit-test-docker: Add linux-headers

Many of our packages like phosphor-networkd require linux kernel
headers to be present on the system. Install these headers so that
the build environment doesn't have to download a copy.

Tested:
    phosphor-networkd no longer downloads linux/ncsi.h

Change-Id: If96ccb40c8c2ae5f7a11f2a49103c832d61a8938
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed