qt5base: fix double-conversion compile for aarch64_be

Fixes [1]:

  ../3rdparty/double-conversion/include/double-conversion/utils.h:81:2: error: #error Target architecture was not detected as supported by Double-Conversion.
  #error Target architecture was not detected as supported by Double-Conversion.

[1] http://autobuild.buildroot.net/results/1fe2be0f26e5b92db57a5cfb5646dd253b731a5c



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>