Unbreak build on FreeBSD

CC @manu. Makes the following succeed:

$ meson -Dudev_kms=false _build
$ ninja -C _build

Clang < 9 also bumps into the following I didn't bother to fix:

../test/xtest.c:64:23: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
    WindowRec root = {0};
                      ^
                      {}