win32: compile mrn_table with extern "C" linkage

This change indended to link with variables declared at ha_mroonga.cpp.
ha_mroonga is compiled with extern "C" linkage, but mrn_table is not, so
Visual Studio can't resolve such external symbols.

Original error message is following:
  mrn_table.obj : error LNK2001: unresolved external symbol "char * mrn_default_parser" (?mrn_default_parser@@3PEADEA) [c:\work\mroonga\mroonga-vs2010-x64\ha_mroonga.vcxproj]
  ...

mrn_table.cpp

@@ -43,6 +43,10 @@
 #define MRN_GROONGA_STR "GROONGA"
 #define MRN_GROONGA_LEN (sizeof(MRN_GROONGA_STR) - 1)
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 extern HASH mrn_open_tables;
 extern pthread_mutex_t mrn_open_tables_mutex;
 extern char *mrn_default_parser;
@@ -923,3 +927,7 @@ void mrn_clear_alter_share(THD *thd)
   }
   DBUG_VOID_RETURN;
 }
+
+#ifdef __cplusplus
+}
+#endif

mrn_table.hpp

@@ -21,6 +21,10 @@
 #ifndef _mrn_table_hpp
 #define _mrn_table_hpp
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <groonga.h>
 
 typedef struct st_mroonga_share
@@ -112,4 +116,8 @@ void mrn_set_bitmap_by_key(MY_BITMAP *map, KEY *key_info);
 st_mrn_slot_data *mrn_get_slot_data(THD *thd, bool can_create);
 void mrn_clear_alter_share(THD *thd);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* _mrn_table_hpp */