package/qt5base: fix double-conversion compile for nios2

Add double-conversion upstream patch to enable compile for nios2.

Fixes:

  http://autobuild.buildroot.net/results/19881951a328ff4df82b5753a23219eb634e86df



  ../3rdparty/double-conversion/include/double-conversion/utils.h:114:2: error: #error Target architecture was not detected as supported by Double-Conversion.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>