package/libplist: fix static link

- libplist depends on libm and optionally on zlib and libiconv (via
  libxml2), so add a patch fixing the build-system that way
- disable shared object build when BR2_PREFER_STATIC_LIB is set

Fixes:
  http://autobuild.buildroot.net/results/9a3/9a364e3d91634a2da2bc481da1dee0ad0e870941/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>