gst/matroska/ebml-read.c: Can't just replace 1LL with 1L here just because MSVC doesn't support it, as it might lead ...

Original commit message from CVS:
* gst/matroska/ebml-read.c: (gst_ebml_read_sint):
Can't just replace 1LL with 1L here just because MSVC doesn't
support it, as it might lead to incorrect results when doing the
bitshifting here. Using GLib's G_GUINT64_CONSTANT() macro to
force a 64-bit constant in a way that all compilers are happy with.