sdltest.cpp missing #include <cstddef>

here is a snippet on what we discussed on IRC that lead to this bug report:

(22:17:15) Turbo: hi guys, I was having problems with "configure -stl" on my system. it seemed like the #include <cstddef> was missing from the config.tests/unix/stl/stltest.cpp file. after that it seemed to have worked fine. any comments?
(22:34:12) peppe: Turbo: which Qt version?
(22:34:20) peppe: Turbo: and which OS / toolchain?
(22:34:40) Turbo: peppe: discovered it in 4.6.3 but the line is also missing from 4.8.0
(22:34:47) Turbo: OS is linux mint debian edition
(22:35:11) Turbo: I'm compiling with gcc 4.6.2
(22:37:25) peppe: Turbo: do you have the exact error you got when configuring (like when running configure -v or something like that)?
(22:41:39) Turbo: peppe: http://pastebin.com/UVVFVLxN
(22:42:16) Turbo: called configure with "-stl -v"
(22:43:05) peppe: Turbo: yep, that's a bug. I can easily fix it in 5.0 but I don't have a 4.8 checkout right now. mind filing a bug report?
(22:45:24) Turbo: okay