installer: use 'mkdir -p' before 'ln -sf'

Fixes the "ln: failed to create symbolic link
'.../usr/lib/libreoffice/sdk/classes/win/unowinreg.dll': No such file or
directory" problem, reported at
http://lists.freedesktop.org/archives/libreoffice/2013-November/057443.html

Change-Id: Ibe7dbbecc670921ca3c372cc9b8655cf2f50aefa