syscall: fix syntax error in mkall.sh

syscall: fix syntax error in mkall.sh

Fix the following error introduced by CL 518627:

    ./mkall.sh: line 370: syntax error near unexpected token `)'
    ./mkall.sh: line 370: `openbsd_riscv64)'

Change-Id: I044563759bf07c94840f2024734d32a0ad663aab
Reviewed-on: https://go-review.googlesource.com/c/go/+/538935
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Heschi Kreinick <heschi@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Bryan Mills <bcmills@google.com>