gdatetime: Mark the usecs as volatile
gdatetime: Mark the usecs as volatile

On i386, we were seeing that this calculation was producing an incorrect
result, probably because usec was being stored in an 80-bit register
before being written back into a 64-bit float in memory. If we mark the
variables as volatile, they are not stored in registers and we avoid
this bug.

endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib