package/asterisk: fix compilation with libsrtp 2.2.0

package/asterisk: fix compilation with libsrtp 2.2.0

Since commit 54797f6489 (package/libsrtp: bump version to 2.2.0),
asterisk now chokes when configured with support for libsrtp, when
that one has also been linked against openssl.

See:
    https://issues.asterisk.org/jira/browse/ASTERISK-27356

Backport the upstream fix for that issue.

Fixes:
    http://autobuild.buildroot.org/results/4e3/4e3b0fd7c31814a8605c00f69cbd45d11800d9e3/
    http://autobuild.buildroot.org/results/c4c/c4c241020d9e7b139b59429498b78243090245a7/
    http://autobuild.buildroot.org/results/1c2/1c266acc0bb0518219979e8e9922f31150716118/

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Bernd Kuhls <bernd.kuhls@t-online.de>
Cc: Gregory Dymarek <gregd72002@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>