C, global variable declarations should not be made in header files.

[tigakub]

ISSUE TEMPLATE ver. 1.2.0

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1. Which DYNAMIXEL SDK version do you use? 3.7.51
2. Which programming language/tool do you use? C, C++, Python
3. Which operating system do you use? Ubuntu 16.04, OS X 10.14
4. Which USB serial converter do you use? USB2AX
5. Which DYNAMIXEL do you use? AX12 AX18 MX28 MX64
6. Have you searched the issue from the closed issue threads? Yes
7. Please describe the issue in detail

packetData (in packet_handler.h), g_used_port_num and g_is_using (both in port_handler.h) are missing the extern prefix resulting in them being defined in multiple object files causing linking to fail.

8. How can we reproduce the issue?

Just try to make any of the c libraries.