devel/msbuild: Avoid MSB1008 build error

The following error:

MSBuild command

/big/ports/usr/ports/devel/msbuild/work/msbuild-0.06/build/../artifacts/mono-msbuild/msbuild "/big/ports/usr/ports/devel/msbuild/work/msbuild-0.06/build/Toolset.proj" /t:restore /m /clp:Summary /warnaserror /v:minimal /bl:"/big/ports/usr/ports/devel/msbuild/work/msbuild-0.06/build/../artifacts/Release-MONO/log/Toolset.binlog"

Microsoft (R) Build Engine version 15.8.38.37692 (xplat-master/3c930fa8 Tue May 8 12:36:23 EDT 2018) for Mono
Copyright (C) Microsoft Corporation. All rights reserved.

MSBUILD : error MSB1008: Only one project can be specified.
Switch: /m

can be fixed by making sure the /m is given the parameter
(number of CPUs to use in parallel).

MSBuild does not seem to parallelize builds currently, but
this fixes the error.

PR: 272961
Approved by: portmgr (build fix blanket)