MDEV-20432 : add MYSQL_PLUGIN_IMPORT

Mroonga does not build on Windows due to log_10_int being used
(indirectly) and n ot properly decorated with MYSQL_PLUGIN_IMPORT

include/my_time.h

@@ -27,7 +27,7 @@
 
 C_MODE_START
 
-extern ulonglong log_10_int[20];
+extern MYSQL_PLUGIN_IMPORT ulonglong log_10_int[20];
 extern uchar days_in_month[];
 
 #define MY_TIME_T_MAX LONG_MAX