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 <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/sim/syscall_abi.hh

@@ -28,6 +28,7 @@
 #ifndef __SIM_SYSCALL_ABI_HH__
 #define __SIM_SYSCALL_ABI_HH__
 
+#include "base/bitfield.hh"
 #include "base/types.hh"
 #include "cpu/thread_context.hh"
 #include "sim/guest_abi.hh"