Add internal.c to unix Makefile's libopusurl.
Add internal.c to unix Makefile's libopusurl.

This shouldn't really be necessary when static linking, but would
 be if someone wanted chang the Makefile to build shared libraries.

youfearm3/opusfile,youfearm3/opusfile,gcp/opusfile,SmartWalkieOrg/opusfile,gcp/opusfile,xiph/opusfile,gcp/opusfile,SmartWalkieOrg/opusfile,xiph/opusfile,SmartWalkieOrg/opusfile,xiph/opusfile