Exclude gdbserver from xwalk_core_library

For the release version of cordova container, there is a license
issue to contain a gdbsever binary in the release binary. Because
the gdbserver is under GPL license.
So exclude gdbserver binary from release version and leave it
for debug version.

BUG=https://crosswalk-project.org/jira/browse/XWALK-635
Merge pull request #1275 from xingnan/gdbserver

Exclude gdbserver from xwalk_core_library