res_srtp: Add support for libsrtp2.x on openSUSE.

Since ASTERISK-27253, no symbols from the header srtp2/crypto_types.h are used
anymore. Therefore, its include statement can be removed. This allows to compile
Asterisk on platforms which do not offer this private header, like openSUSE.

ASTERISK-27733

Change-Id: I25c5cb8fa966043d1506ebef449e5a724412b4b6