client_errors symbol should be exported

The client_errors global array is defined in libmariadb/errmsg.c, and accessible via the macros defined in include/errmsg.h.

It appears to be part of the public API, since code in the PHP mysqli extension accesses it directly via the ER() macros.

However, client_errors is not in libmariadb_exports.def or version_script.txt, which means that the symbol is not accessible when linking against the DSO/DLL.

I verified that adding client_errors to the list of symbols under global: in version_script.txt resolves the issue for me on a Linux system. Not sure about Win32 but looks like the analogous simple change in libmariadb_exports.def would take care of this assuming the same issue is present there.