httping: fix math library linking

Add the math library directive '-lm' to linker options at the end.
The order of the math library directive '-lm' matters.

Fixes:

  http://autobuild.buildroot.net/results/843/84382290696e72c23995f5ed020ad5c157817012/



Signed-off-by: Yuvaraj Patil <yuvaraj.patil@wipro.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>