Enusure that my_global.h is included first

- Added sql/mariadb.h file that should be included first by files in sql
  directory, if sql_plugin.h is not used (sql_plugin.h adds SHOW variables
  that must be done before my_global.h is included)
- Removed a lot of include my_global.h from include files
- Removed include's of some files that my_global.h automatically includes
- Removed duplicated include's of my_sys.h
- Replaced include my_config.h with my_global.h

plugin/auth_gssapi/sspi_client.cc

@@ -27,11 +27,10 @@ POSSIBILITY OF SUCH DAMAGE.
 */
 
 #define SECURITY_WIN32
+#include <my_global.h>
 #include <windows.h>
 #include <sspi.h>
 #include <SecExt.h>
-#include <stdarg.h>
-#include <stdio.h>
 
 #include <mysql/plugin_auth.h>
 #include <mysql.h>
@@ -180,4 +179,4 @@ cleanup:
     FreeCredentialsHandle(&cred);
   free(out);
   return ret;
-}
+}