db_mysql: replaced my_bool with bool

- mysql 8.0.1 removed my_bool and suggests use of bool:
  * https://dev.mysql.com/doc/relnotes/mysql/8.0/en/news-8-0-1.html
- use stdbool.h, it defines bool in C99, which should be old enough to
be everywhere, otherwise it may require some ifdefs to get it for older
but still alive systems
- reported by GH #1602

src/modules/db_mysql/km_my_con.c

@@ -29,6 +29,7 @@
 #include "km_my_con.h"
 #include "km_db_mysql.h"
 #include <mysql.h>
+#include <stdbool.h>
 #include "../../core/mem/mem.h"
 #include "../../core/dprint.h"
 #include "../../core/ut.h"
@@ -44,7 +45,7 @@ struct my_con* db_mysql_new_connection(const struct db_id* id)
 	char *host, *grp, *egrp;
 	unsigned int connection_flag = 0;
 #if MYSQL_VERSION_ID > 50012
-	my_bool rec;
+	bool rec;
 #endif
 
 	if (!id) {

src/modules/db_mysql/my_fld.h

@@ -29,12 +29,13 @@
 #include "../../lib/srdb2/db_drv.h"
 #include "../../lib/srdb2/db_fld.h"
 #include <mysql.h>
+#include <stdbool.h>
 
 struct my_fld {
 	db_drv_t gen;
 
 	char* name;
-	my_bool is_null;
+	bool is_null;
 	MYSQL_TIME time;
 	unsigned long length;
 	str buf;