Fix PowerShell Write-Error multi-line argument

This is a follow-up of d04c84e7 and fixes the error message that is
displayed if someone tries to run `contrib/win-installer/build.ps1` but
doesn't have the .NET SDK installed.

Signed-off-by: Mario Loriedo <mario.loriedo@gmail.com>

contrib/win-installer/build.ps1

@@ -103,8 +103,8 @@ Write-Host 'Checking requirements (dotnet and wix)'
 
 # Check if .NET SDK is installed
 if (! (Get-Command 'dotnet' -errorAction SilentlyContinue)) {
-    Write-Error "Required dep `".NET SDK`" is not installed. " `
-        + 'Please install it from https://dotnet.microsoft.com/download'
+    Write-Error ("Required dep `".NET SDK`" is not installed. " `
+        + 'Please install it from https://dotnet.microsoft.com/download')
     Exit 1
 }