package/libupnp18: fix static linking with mpd

- Add a call to PKG_CHECK_MODULES in configure.ac to get openssl
  libraries and its dependencies if openssl support is enabled
- Add OPENSSL_LIBS to libupnp.pc.in so that applications linking with
  pupnp (such as mpd) will be able to retrieve openssl libraries

Fixes:
 - http://autobuild.buildroot.org/results/a4148e516070b79816769f3443fc24d6d8192073



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>