Fix compilation of ODBC-specific SOCI header with new include paths.

If only soci/include directory is in the compiler include path, including
<soci-backend.h> doesn't compile, as it should be included as
<soci/soci-backend.h> or, as this patch does, "../soci-backend.h", which is
uglier but simpler as it doesn't require changing any SOCI makefiles.