libupnpp: fix static linking issue with libcurl

Fixes:
http://autobuild.buildroot.net/results/f38/f383bf4d7572a7dd59d382cd06a9baeb37e9161c/
http://autobuild.buildroot.net/results/d93/d934871d1b192706cd9163715c5ebcd96deff2b8/



Libcurl optionally links to a number of libraries that have to be taken into
consideration when linking statically.

Use pkg-config to get the correct libcurl link line, and also use it for
libupnp instead of the hardcoded list.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>