mysql: disable maintainer mode

The maintainer mode enables additional gcc warnings, including
-Werror, which causes build failures with certain toolchains. Since we
generally don't enable -Werror in Buildroot, it doesn't make much
sense to have it enabled for MySQL.

Fixes:

  http://autobuild.buildroot.net/results/6b6/6b61f90d1a36ba43ff568933bb74975128380516/



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