package/libsrtp: fix static build with openssl

libsrtp forget to link with zlib for static build when openssl
support is requested.

The issue was reported upstream by Vicente Olivert Riera:
https://github.com/cisco/libsrtp/issues/97

Backport upstream patch for the upcoming libsrtp 1.5.2.
Since the libsrtp package already use AUTORECONF, apply only
the changes for configure.in.

Fixes:
http://autobuild.buildroot.net/results/a71/a71d85a5bb9af2ee82a8e61054025bceb03dbdeb/



Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>