Changes

cosmetic: add copy of mgcp_common.h to .gitignore

The build process copies include/osmocom/mgcp/mgcp_common.h to
include/osmocom/mgcp_client/mgcp_common.h; the former is a true
source while the latter is a build-generated file.  The copied
file needs to be included in .gitignore, otherwise it adds
noise to git status output.

Change-Id: Icf0e3f724564d87eef9351b22de7a3d3aa7e4264