spirit/symbols.ipp pragma warning fix

[jmihalicza]

The original version sets the default setting back at the 
end of the file so the following code doesn't work as 
expected:
#pragma warning (disable:4355)
#include <boost/spirit/....> // some file that includes 
symbols.ipp
// here 4355 may be enabled according to the default 
settings (warning level)

The fix uses push/pop

[Hartmut Kaiser]

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user_id=353975

I've applied the proposed patch.

Thanks and regards Hartmut