mysql: patch for Windows and FreeBSD (#6703)

vlib/mysql/_cdefs.c.v

@@ -1,11 +1,14 @@
 module mysql
 
+[typedef]
 struct C.MYSQL {
 }
 
+[typedef]
 struct C.MYSQL_RES {
 }
 
+[typedef]
 struct C.MYSQL_FIELD {
 	name             byteptr // Name of column
 	org_name         byteptr // Original column name, if an alias

vlib/mysql/_cdefs_nix.c.v

@@ -0,0 +1,5 @@
+module mysql
+
+#pkgconfig mysqlclient
+#include <mysql.h> # Please install the mysqlclient development headers
+

vlib/mysql/_cdefs_windows.c.v

@@ -0,0 +1,5 @@
+module mysql
+
+#flag windows -I@VROOT/thirdparty/mysql/include
+#flag windows @VROOT/thirdparty/mysql/lib/libmysql.dll
+#include <mysql.h> # Please install https://dev.mysql.com/downloads/installer/ , then put the include/ and lib/ folders in thirdparty/mysql

vlib/mysql/mysql.v

@@ -1,8 +1,5 @@
 module mysql
 
-#flag -lmysqlclient
-#flag linux -I/usr/include/mysql
-#include <mysql.h>
 // TODO: Documentation
 pub struct Connection {
 mut: