Fixing a build issue. The function innobase_convert_to_system_charset()

is included only in the builtin InnoDB, and it is missed in InnoDB
plugin.  Adding this function in InnoDB plugin as well.

storage/innodb_plugin/handler/ha_innodb.cc

@@ -1052,6 +1052,23 @@ innobase_convert_from_id(
 	strconvert(cs, from, system_charset_info, to, (uint) len, &errors);
 }
 
+/**********************************************************************
+Converts an identifier from my_charset_filename to UTF-8 charset. */
+extern "C"
+uint
+innobase_convert_to_system_charset(
+/*===============================*/
+	char*		to,	/* out: converted identifier */
+	const char*	from,	/* in: identifier to convert */
+	ulint		len,	/* in: length of 'to', in bytes */
+	uint*		errors)	/* out: error return */
+{
+	CHARSET_INFO*	cs1 = &my_charset_filename;
+	CHARSET_INFO*	cs2 = system_charset_info;
+
+	return(strconvert(cs1, from, cs2, to, len, errors));
+}
+
 /******************************************************************//**
 Compares NUL-terminated UTF-8 strings case insensitively.
 @return	0 if a=b, <0 if a<b, >1 if a>b */