libmicrohttpd: bump version

The old 0.4.2 version doesn't seem to be available on the GNU mirrors
anymore, so change to the slightly-less-ancient 0.4.6 version.

We're not moving to the new 0.9.x series yet, as that one needs an
external libgnutls for https support, which we don't have a BR
package for yet.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>