Botan: Fix MSVC warning

botan.cpp:35140: warning: C4297: 'Botan::bigint_sub2_rev' : function assumed not to throw an exception but does
The function is extern "C" and /EHc was specified

-EHsc (specified in Qt mkspecs) means that the compiler can assume that
extern "C" functions do not throw exceptions.

-EHs is equivalent, only without this assumption

Change-Id: I5eae79bfbd647c296626c03636775f6057faf385
Reviewed-by: Christian Kandeler <christian.kandeler@digia.com>