package/libupnpp: needs at least gcc 4.6

Version 0.13.1 of libupnpp uses range-based for loops. This C++11 feature
requires at least gcc 4.6.

Fixes:
http://autobuild.buildroot.net/results/aa9/aa93ff96305a3b201636bea65f1cfdb6db6d3dfb/
http://autobuild.buildroot.net/results/d8b/d8b96987a5336a64857817ec9831b0fb12867e7c/
http://autobuild.buildroot.net/results/408/40807fe84f67961817b6592a6df243c036cf3b4b/
http://autobuild.buildroot.net/results/408/40807fe84f67961817b6592a6df243c036cf3b4b/


.. and many more

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>