[Rd] unexported symbols in libR.so of r-devel

[Na Li]

Thanks to Matthias Burger, I came to know that in r-devel, a lot of symbols
are no longer exported in libR.so, which breaks the package rpvm since it
calls these functions in serialize.c.

R_InitInPStream
R_InitOutPStream

Since the change is fairly recent (earlier this month), I guess the core team
will be making further changes.  In any case, please add these two to the
exported list.

Thanks,

Michael