xenpaging: remove _XOPEN_SOURCE

xenpaging: remove _XOPEN_SOURCE

The _XOPEN_SOURCE define was breaking the compilation under NetBSD.
I've removed it becasue it is not necessary.

Error message:

gcc  -O1 -fno-omit-frame-pointer -m64 -g -fno-strict-aliasing
-std=gnu99 -Wall -Wstrict-prototypes -Wno-unused-value
-Wdeclaration-after-statement   -D__XEN_TOOLS__ -MMD -MF
.xenpaging.o.d -fno-optimize-sibling-calls
-I/root/xen-04102011/tools/xenpaging/../../tools/libxc
-I/root/xen-04102011/tools/xenpaging/../../tools/include
-I/root/xen-04102011/tools/xenpaging/../../tools/xenstore
-I/root/xen-04102011/tools/xenpaging/../../tools/include -Werror
-Wno-unused -g  -c -o xenpaging.o xenpaging.c  -I/usr/xen42/include
-I/usr/include
cc1: warnings being treated as errors
xenpaging.c: In function 'xenpaging_init':
xenpaging.c:333: warning: implicit declaration of function 'asprintf'

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>