Makefile: Fix typo for MOCK_TPM

CL:1900560 broke MOCK_TPM due to a typo in the Makefile. This patch
fixes it.

BRANCH=None
BUG=None
TEST=None

Change-Id: Ifd23ad764029b72af3fb03cf9dd52faefa586a80
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/1922491
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Joel Kitching <kitching@chromium.org>
1 file changed