package/boost: add patch to fix undeclared isnan issue

Add patch from upstream [1] to fix build error:
  ./boost/math/special_functions/fpclassify.hpp:137:75: error: '::isnan' has not been declared

Fixes:
http://autobuild.buildroot.net/results/6c7/6c72c189cd029fb6c81798f5443fab331358bb90/
http://autobuild.buildroot.net/results/b70/b709fc201c9df8f76163e2a620da32362abe5288/
http://autobuild.buildroot.net/results/99b/99bdbc5622dfd0ac369611ec07dd2a8a8fe41ce8/
http://autobuild.buildroot.net/results/d2b/d2b270787fc20e45c839d5670af55e47723aef2a/
and many more.

Backported from: fbd1393858719c7bda7d251f742950c1bc691ea8

[1] https://github.com/boostorg/math/commit/fbd1393858719c7bda7d251f742950c1bc691ea8



Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>