Fix out-of-source tree builds.
Fix out-of-source tree builds.

The pkg-config file is generated in the build directory, so we need to
copy it from there.

cuijinquan/libsrtp,yarrcc/libsrtp-ios,persmule/libsrtp,yarrcc/libsrtp-ios,cuijinquan/libsrtp,yarrcc/libsrtp-ios,persmule/libsrtp,cuijinquan/libsrtp,persmule/libsrtp