Fix CONFIG missing link_pkgconfig on Windows
https://bugs.webkit.org/show_bug.cgi?id=106647

Some libraries (e.g. libxslt, libxml2 and sqlite3) fail to link
properly on Windows because they are added to PKGCONFIG but
CONFIG does not contain link_pkgconfig.

This is because link_pkgconfig is added in unix/default_pre.prf
which is only used for UNIX platforms. To fix this, move it to
the general default_pre.prf.

Patch by Jonathan Liu <net147@gmail.com> on 2013-01-13
Reviewed by Simon Hausmann.

* qmake/mkspecs/features/default_pre.prf:
* qmake/mkspecs/features/unix/default_pre.prf:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@139570 268f45cc-cd09-0410-ab3c-d52691b4dbfc