package/cURL: fix static link whith openSSL

When openSSL is selected, cURL is configured to use it.

But in this case, the libcurl.pc file /forgets/ to require link
against -ldl.

This can happen, for example, when BR2_PREFER_STATIC_LIB is not set,
but an executable wants to be linked statically (for various reasons
which are irrelevant here).

Fix that by appending a 'Requires: openssl' line to libcurl.pc.in,
but only if openSSL is enabled.

As suggested by Arnout, do it in a post-patch hook, rather as a
post-install hook.

Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>