Fix warning issued by GCC 4.8 in oics/tinyxml.cpp

Fix warning issued by GCC 4.8 (and as far I remember 4.7 too)
about not supported "%lf" format string:

openmw/extern/oics/tinyxml.cpp: In member function ‘void TiXmlAttribute::SetDoubleValue(double)’:
openmw/extern/oics/tinyxml.cpp:1269:50: warning: ISO C++98 does not support the ‘%lf’ gnu_printf format [-Wformat=]
   TIXML_SNPRINTF( buf, sizeof(buf), "%lf", _value);
                                                  ^

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>