gdb: un-share set_inferior_cwd declaration

gdb: un-share set_inferior_cwd declaration

The declaration of set_inferior_cwd is currently shared between gdb and
gdbserver, in gdbsupport/common-inferior.h.  It doesn't need to be, as
set_inferior_cwd is not called from common code.  Only get_inferior_cwd
needs to.

The motivation for this is that a future patch will change the prototype
of set_inferior_cwd in gdb, and I don't want to change it for gdbserver
unnecessarily.  I see this as a good cleanup in any case, to reduce to
just the essential what is shared between GDB and GDBserver.

Change-Id: I3127d27d078f0503ebf5ccc6fddf14f212426a73