kernel32: Work around safety check on siglongjmp

[Maarten Lankhorst]

Since fibres have their own stack, the paranoid version of siglongjmp 
enabled with _FORTIFY_SOURCE thinks that the stack pointer is garbled. 
Since ubuntu 10.4, fedora, and gentoo are all affected, just undef 
_FORTIFY_SOURCE before including setjmp.h here.

This fixes bug 21405
---
The other calls of longjmp are unaffected, since wine either uses its 
own functions, or uses it in a way that doesn't cause stack switching.
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