package/asterisk: libsrtp needs openssl

package/asterisk: libsrtp needs openssl

Fixes configure error

checking for the ability of -lsrtp2 to be linked in a shared object... yes
checking for srtp_crypto_policy_set_aes_cm_256_hmac_sha1_80 in -lsrtp2... yes
checking for srtp_crypto_policy_set_aes_cm_192_hmac_sha1_80 in -lsrtp2... no
checking for srtp_crypto_policy_set_aes_gcm_128_8_auth in -lsrtp2... no
checking for srtp_shutdown in -lsrtp2... yes
checking for srtp2/srtp.h... (cached) yes
configure: WARNING: ***
configure: WARNING: *** OpenSSL required when using libsrtp2, checking for libsrtp instead.
configure: WARNING: ***

using this defconfig:

BR2_PACKAGE_LIBSRTP=y
BR2_PACKAGE_ASTERISK=y

Please note that openssl support in libsrtp is not available for static
builds:
https://git.buildroot.net/buildroot/tree/package/libsrtp/libsrtp.mk#n27

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>