package/qt5base: fix double-conversion compile for riscv

Fixes [1], [2]:

  ../3rdparty/double-conversion/include/double-conversion/utils.h:83:2: error: #error Target architecture was not detected as supported by Double-Conversion.

[1] http://autobuild.buildroot.net/results/8bdb6a2b56f6ea96649184e5fef6ce2c56b9ec2b
[2] http://autobuild.buildroot.net/results/63ba267674b424786bb10c97c565f4306147f95a



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