build: Add -Wdeclaration-after-statement.

build: Add -Wdeclaration-after-statement.

No code in the tree has declarations after statements, so this
change produces no new warnings. It makes sure, however, that we do
not introduce such declarations in the future. The option was not
enabled only because old gcc versions do not support it. Since we
require gcc-4.2.x or newer these days, we can count on the option
and enable it unconditionally.