unix: re-add vm.* sysctl strings on openbsd/amd64

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>

unix/mksysctl_openbsd.pl

@@ -32,6 +32,7 @@ my @headers = qw (
 	sys/sem.h
 	sys/shm.h
 	sys/vmmeter.h
+	uvm/uvmexp.h
 	uvm/uvm_param.h
 	uvm/uvm_swap_encrypt.h
 	ddb/db_var.h

unix/zsysctl_openbsd_amd64.go

@@ -254,4 +254,17 @@ var sysctlMib = []mibentry{
 	{"net.mpls.ttl", []_C_int{4, 33, 2}},
 	{"net.pflow.stats", []_C_int{4, 34, 1}},
 	{"net.pipex.enable", []_C_int{4, 35, 1}},
+	{"vm.anonmin", []_C_int{2, 7}},
+	{"vm.loadavg", []_C_int{2, 2}},
+	{"vm.maxslp", []_C_int{2, 10}},
+	{"vm.nkmempages", []_C_int{2, 6}},
+	{"vm.psstrings", []_C_int{2, 3}},
+	{"vm.swapencrypt.enable", []_C_int{2, 5, 0}},
+	{"vm.swapencrypt.keyscreated", []_C_int{2, 5, 1}},
+	{"vm.swapencrypt.keysdeleted", []_C_int{2, 5, 2}},
+	{"vm.uspace", []_C_int{2, 11}},
+	{"vm.uvmexp", []_C_int{2, 4}},
+	{"vm.vmmeter", []_C_int{2, 1}},
+	{"vm.vnodemin", []_C_int{2, 9}},
+	{"vm.vtextmin", []_C_int{2, 8}},
 }