oracle-mysql: fix build with gcc 7.x

This adjusts an existing patch to also fix gcc 7.x build issues on the
server side of MySQL.

Fixes:

  http://autobuild.buildroot.net/results/9e3a0b5b8f2506fb2ce2fcc3d5d3a494a14439cb



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>