[opts files] Remove -Wno-packed-bitfield-compat from GXX_WARNING_OPTS

because new gcc do not warn about backed bitfields for our code.
(reviewed by bernd)


git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10095 2fb0f335-1f38-0410-981e-8018bf24f1b0