vfiouserbuild: fix build when using dash

When /bin/sh is dash (such as on Ubuntu), the build now fails with:

/bin/sh: 2: Syntax error: Missing '))'

Avoid this problem by adding a space between the opening brackets.

Signed-off-by: John Levon <john.levon@nutanix.com>
Change-Id: I420788bcfaacfafc5f060fb56866edcfe2a0f1a8
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/14392


Reviewed-by: Jonathan Davies <jonathan.davies@nutanix.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>