boost: fix build for sparc

Using upstream patch, fixes:
http://autobuild.buildroot.net/results/dd0/dd032aa7b387f3ba5b25bffcaf833ba0ea9ba199/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>