These sysctl strings were lost in CL 114075 which updated openbsd/amd64
to OpenBSD 6.3. Re-add them by adding uvm/uvmexp.h to mksysctl_openbsd.pl.
The defines for the VM_* constants were moved to this header in
OpenBSD 6.3.

Change-Id: If4291380d6d8235485d91c4442076dd03485bc2c
Reviewed-on: https://go-review.googlesource.com/c/139277
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>