LU-14083 build: Don't overwrite KBUILD_EXTRA_SYMBOLS

LU-14083 build: Don't overwrite KBUILD_EXTRA_SYMBOLS

The gnilnd requires that KBUILD_EXTRA_SYMBOLS be populated with the
symbols for some of its dependencies. Don't overwrite
KBUILD_EXTRA_SYMBOLS for the kernel tests.

Test-Parameters: trivial
HPE-bug-id: LUS-9448
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Ie10d3be0078e163f3e36f1c880d87ebc185c49c7
Reviewed-on: https://review.whamcloud.com/40435
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>