9pfsd: allow building with old glibc

Neither pread() / pwrite() nor O_DIRECTORY are available from glibc
2.11.3 headers without defining (e.g.) _GNU_SOURCE. Supplying the
definition unconditionally shouldn't be a problem, seeing that various
other tools/ components do so, too.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

tools/9pfsd/Makefile

@@ -5,7 +5,7 @@
 XEN_ROOT = $(CURDIR)/../..
 include $(XEN_ROOT)/tools/Rules.mk
 
-CFLAGS += $(PTHREAD_CFLAGS)
+CFLAGS += $(PTHREAD_CFLAGS) -D_GNU_SOURCE
 LDFLAGS += $(PTHREAD_LDFLAGS)
 
 TARGETS := xen-9pfsd