Remove -Wdeclaration-after-statement from default warning set

We're going to treat this as a code style issue to be considered in review instead of a code portability issue that should always be warned about, now that all the compilers we support can handle variable declarations after statements.

See the xorg-devel discussion starting at: https://lists.x.org/archives/xorg-devel/2024-July/059279.html and continuing at: https://lists.x.org/archives/xorg-devel/2024-August/059289.html.