Fix Linux-only check in prebuilt audit2allow

Fix the following error:

$ audit2allow external/selinux/prebuilts/bin/audit2allow: 4: /external/selinux/prebuilts/bin/audit2allow: [[: not found
audit2allow is only supported on linux

Bug: 21894097
Change-Id: Ic7df339daa3a8db891fd8de3584da10c8bb2f4d5
1 file changed