sim: Add a missing include to sim/syscall_abi.hh.
sim: Add a missing include to sim/syscall_abi.hh.

This must have been included indirectly in the past.

Change-Id: I8be3a11ca386e420f04d57e51a89c47e6a747e18
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36616
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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