libcurl: specify capath

Since openssl's path is '/etc/ssl/' (specified in our openssl package),
we should also make sure that's what curl is using.

Previously, it's hasn't been specified, which means it changes depending
on the host system where it's compiled.

Signed-off-by: Shawn J. Goff <shawn7400@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>