Add multiarch metadata to srtp again

[Derek Gasaway]

On my current version of Ubuntu (17.10) and packages libsrtp0 and libsrtp0-dev at version 1.4.5~20130609~dfsg-2ubuntu1, these packages do not actually have multiarch support.

I was able to download the package source, add Multi-Arch: same to both, and build and install libsrtp0 on i386 and amd64 simultaneously, however, and likely could have also installed both versions of libsrtp0-dev.

Also, configure.in has a check for cross-compilation, which prevents using /dev/urandom as a random source, and caused the tests to fail, so when I cross-built the package I forced set DEV_URANDOM=/dev/urandom.

According to the log (and a previous bug report https://bugs.launchpad.net/ubuntu/+source/srtp/+bug/1508634) multiarch support was merged in the past, so I'm curious why it got removed again.