UI/installer: Add quotes around UninstallString

This follows recommended practice from NSIS documentation.

This fixes issue #7935.

UI/installer/mp-installer.nsi

@@ -406,7 +406,7 @@ Section -FinishSection
 
 	WriteRegStr HKLM "Software\${APPNAME}" "" "$INSTDIR"
 	WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\${APPNAME}" "DisplayName" "${APPNAME}"
-	WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\${APPNAME}" "UninstallString" "$INSTDIR\uninstall.exe"
+	WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\${APPNAME}" "UninstallString" "$\"$INSTDIR\uninstall.exe$\""
 	WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\${APPNAME}" "ProductID" "d16d2409-3151-4331-a9b1-dfd8cf3f0d9c"
 !ifdef INSTALL64
 	WriteRegStr HKLM "Software\Microsoft\Windows\CurrentVersion\Uninstall\${APPNAME}" "DisplayIcon" "$INSTDIR\bin\64bit\obs64.exe"