Fix build warning with MinGW32 and CPython 2.4 and 2.5

To build the C extensions on Windows for CPython 2.4 and 2.5 you need Visual Studio 2003 (which nobody has anymore) or MinGW32. Using MinGW32 we have the following build warning:

bson/_cbsonmodule.c: In function '_write_element_to_buffer':

bson/_cbsonmodule.c:897:13: warning: implicit declaration of function '_scprintf'

This doesn't ultimately matter since the resulting shared object still functions properly, but we should fix it anyway.