isalbuild: Fix highlight context in Makefile.

Here should be tab. Though it doesn't impact
on the result. But it's a fault.

Signed-off-by: yidong0635 <dongx.yi@intel.com>
Change-Id: I0f127c6210c4e801adb8b1cb8fadc1d4554b99a4
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/12752


Community-CI: Broadcom CI <spdk-ci.pdl@broadcom.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: GangCao <gang.cao@intel.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@nvidia.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>