[master] Include <mysql.h> rather than <mysql/mysql.h>

This prevents compilation failure on OS-X. Reviewed and approved
on jabber.

src/lib/dhcpsrv/mysql_connection.h

@@ -17,7 +17,7 @@
 
 #include <dhcpsrv/database_connection.h>
 #include <boost/scoped_ptr.hpp>
-#include <mysql/mysql.h>
+#include <mysql.h>
 #include <vector>
 
 namespace isc {

src/lib/dhcpsrv/mysql_host_data_source.h

@@ -21,7 +21,7 @@
 
 #include <boost/scoped_ptr.hpp>
 #include <boost/utility.hpp>
-#include <mysql/mysql.h>
+#include <mysql.h>
 
 namespace isc {
 namespace dhcp {

src/lib/dhcpsrv/mysql_lease_mgr.h

@@ -21,7 +21,7 @@
 
 #include <boost/scoped_ptr.hpp>
 #include <boost/utility.hpp>
-#include <mysql/mysql.h>
+#include <mysql.h>
 
 #include <time.h>