tools: avoid redefinition of typedefs

tools: avoid redefinition of typedefs

When splitting out various functionality from libxc into tools/libs/*
I attempted to make it possible to avoid callers being unnecessarily
exposed to the xentoollog interface by providing a typedef of the
xentoollog_logger handle in each of the headers.

However such typedefs are not allowed in C, instead it is necessary to
forward declare the struct and then use the struct xentoollog_logger
variant in the prototypes.

It appears that older gcc (e.g. 4.4) complains about this issue while
newer ones (e.g. 4.9) are more tolerant unless -pedantic-errors is
used, this was a deliberate change
https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=ce3765bf44e49ef0568a1ad4a0b7f807591d6412

As well as tools/libs/* it is also now necessary to give libvchan the
same treatment, since it previously inhereted the typedef via one of
tools/libs/*.

Reported-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>