xwayland: Fix order of wlr_xwayland_surface_size_hints fields

wlr_xwayland_surface_size_hints is memcpy'd from xcb_size_hints_t and therefore the struct layout needs to match xcb_size_hints_t exactly.

The base_width/base_height fields have been in the wrong location since commit 12b9b1a4 ("Copy xcb_icccm structs into wlroots").

See https://code.woboq.org/qt5/include/xcb/xcb_icccm.h.html (search "xcb_size_hints_t")