msbuild: add pch for dolphinqt

[shuffle2]

since this now includes Qt headers it's better than only using
the vanilla DolphinLib one alone (previous behavior before Qt6)

this PR:

c:\src\dolphin>rmdir /s/q build\x64\Release\Dolphin && powershell measure-command { msbuild -v:m -m -p:Configuration=Release,Platform=x64 Source\Core\DolphinQt\DolphinQt.vcxproj }


Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 23
Milliseconds      : 777
Ticks             : 237779051
TotalDays         : 0.000275207234953704
TotalHours        : 0.00660497363888889
TotalMinutes      : 0.396298418333333
TotalSeconds      : 23.7779051
TotalMilliseconds : 23777.9051

master:

c:\src\dolphin>rmdir /s/q build\x64\Release\Dolphin && powershell measure-command { msbuild -v:m -m -p:Configuration=Release,Platform=x64 Source\Core\DolphinQt\DolphinQt.vcxproj }


Days              : 0
Hours             : 0
Minutes           : 0
Seconds           : 36
Milliseconds      : 721
Ticks             : 367217774
TotalDays         : 0.000425020571759259
TotalHours        : 0.0102004937222222
TotalMinutes      : 0.612029623333333
TotalSeconds      : 36.7217774
TotalMilliseconds : 36721.7774