Fix build errors and linker warnings on OS X

Summary:
Fix compile errors and linker warnings in the debuggers subdirectory when build on OS X.

• Use #include <signal.h> rather than #include "sys/sys/signal.h"
• Fix visibility argument settings on kdevdebuggercommon to avoid linker warnings

Detailed discuss can be found on mailing list 1 2

Test Plan: it now builds without any errors or linker warnings, tested by René

Reviewers: apol, KDevelop, rjvbb

Reviewed By: rjvbb

Subscribers: kdevelop-devel, rjvbb

Differential Revision: https://phabricator.kde.org/D2039