package/uhd: needs boost math

package/uhd: needs boost math

Lots of uhd components (e.g. examples, USRP1, USRP2) needs boost math
(i.e. https://github.com/EttusResearch/uhd/search?q=boost%3A%3Amath)
resulting in build failures since commit
c577eac16eaae515973faf3013da197516bfd391

Fixes:
 - http://autobuild.buildroot.org/results/70f6db101c9d35cdd88da602a863ddf35706fd7d
 - http://autobuild.buildroot.org/results/5b28591b4c4a7ae4cc6d428c42d96db138ef3ee7

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>