package/icu: fix double conversion compile for xtensa

package/icu: fix double conversion compile for xtensa

Fixes [1]:

  In file included from double-conversion.h:42,
                   from number_decimalquantity.cpp:19:
  double-conversion-utils.h:121:2: 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/fc331ae8e2484be6818f4fc334d3f8c9658fffdc

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>