libmicrohttpd: fix static linking with gnutls

Fixes http://autobuild.buildroot.net/results/f5a/f5a135647867ca98ce6189bb343a631ce6a47e23/



gnutls links to a number of libraries depending on configuration (pthread,
iconv, ..), so we need to link against those as well when statically
linking.

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