tools/xenstore: mark variable in header as extern

tools/xenstore: mark variable in header as extern

This patch fix "multiple definition of `xprintf'" (or xgt_handle)
build error with GCC 10.1.0.

These are the error reported:
    gcc xs_tdb_dump.o utils.o tdb.o talloc.o      -o xs_tdb_dump
    /usr/bin/ld: utils.o:./utils.h:27: multiple definition of `xprintf'; xs_tdb_dump.o:./utils.h:27: first defined here
    [...]
    gcc xenstored_core.o xenstored_watch.o xenstored_domain.o xenstored_transaction.o xenstored_control.o xs_lib.o talloc.o utils.o tdb.o hashtable.o xenstored_posix.o      -lsystemd   -Wl,-rpath-link=... ../libxc/libxenctrl.so -lrt  -o xenstored
    /usr/bin/ld: xenstored_watch.o:./xenstored_core.h:207: multiple definition of `xgt_handle'; xenstored_core.o:./xenstored_core.h:207: first defined here
    /usr/bin/ld: xenstored_domain.o:./xenstored_core.h:207: multiple definition of `xgt_handle'; xenstored_core.o:./xenstored_core.h:207: first defined here
    /usr/bin/ld: xenstored_transaction.o:./xenstored_core.h:207: multiple definition of `xgt_handle'; xenstored_core.o:./xenstored_core.h:207: first defined here
    /usr/bin/ld: xenstored_control.o:./xenstored_core.h:207: multiple definition of `xgt_handle'; xenstored_core.o:./xenstored_core.h:207: first defined here
    /usr/bin/ld: xenstored_posix.o:./xenstored_core.h:207: multiple definition of `xgt_handle'; xenstored_core.o:./xenstored_core.h:207: first defined here

A difference that I noticed with earlier version of the build chain is
that before, I had:
    $ nm xs_tdb_dump.o | grep xprintf
    0000000000000008 C xprintf
And now, it's:
    0000000000000000 B xprintf
With the patch apply, the symbol isn't in xs_tdb_dump.o anymore.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>