PCbuild: Add -q option to svn export (GH-535)

Without this option, AppVeyor log is too unreadable.
diff --git a/PCbuild/get_externals.bat b/PCbuild/get_externals.bat
index 98a755d8..3a2656e 100644
--- a/PCbuild/get_externals.bat
+++ b/PCbuild/get_externals.bat
@@ -66,7 +66,7 @@
         echo.%%e already exists, skipping.

     ) else (

         echo.Fetching %%e...

-        svn export %SVNROOT%%%e

+        svn export -q %SVNROOT%%%e

     )

 )