libxenstat: avoid build race

libxenstat: avoid build race

Olaf reported observing

xenstat_qmp.c:26:10: fatal error: _paths.h: No such file or directory
.../tools/libs/stat/../../../tools/Rules.mk:153: xenstat_qmp.opic] Error 1

Obviously _paths.h, when included by any of the sources, needs to be
created in advance of compiling any of them, not just when compiling
into non-PIC objects.

Reported-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Wei Liu <wl@xen.org>