libevent: drop openssl static linking workaround

Commit f29bb169 (libevent: fix openssl detection when statically linking)
forced libevent to link against zlib when statically linking against openssl
as the configure script failed to do this.

Since then libevent has been bumped, and it now uses pkg-config to figure
out how to link with openssl, so ensure pkg-config is available and drop the
now-unneeded workaround.

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