cmake: add patch to fix Qt mkspecs detection

The FindQt4 macros that search the mkspecs directory had a bug that
prevented the mkspecs directory from being found, causing build
failures of the qjson and grantlee.

Fixes:

  http://autobuild.buildroot.org/results/75b/75b45da597c12034baad50492c0c8150dc1bf0aa/build-end.log
  http://autobuild.buildroot.org/results/3b9/3b9d022b02c4497efed178f35c069ea99a7d3eef/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>