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>

src/libs/3rdparty/botan/botan.pri

@@ -35,6 +35,7 @@ win32 {
         BOTAN_HAS_MUTEX_WIN32
 
     win32-msvc* {
+        QMAKE_CXXFLAGS_EXCEPTIONS_ON = -EHs
         QMAKE_CXXFLAGS += -wd4251 -wd4290 -wd4250
         DEFINES += BOTAN_BUILD_COMPILER_IS_MSVC BOTAN_TARGET_OS_HAS_GMTIME_S _SCL_SECURE_NO_WARNINGS
     } else {