PCbuild: Add -q option to svn export (GH-539)

Without this option, AppVeyor log is too unreadable.
(cherry picked from commit 8886d5f3)

PCbuild/get_externals.bat

@@ -69,7 +69,7 @@ for %%e in (%libraries%) do (
         echo.%%e already exists, skipping.
     ) else (
         echo.Fetching %%e...
-        svn export %SVNROOT%%%e
+        svn export -q %SVNROOT%%%e
     )
 )