nsslinktest: also build virAtomic.h

When build for architecture that don't use gcc atomic ops but pthread,
it fails to build for armel:

| ../tools/nss/.libs/libnss_libvirt_impl.a(libvirt_nss_la-virobject.o): In function `virClassNew':
| /buildarea2/kkang/builds/qemuarm-Aug03/bitbake_build/tmp/work/armv5e-wrs-linux-gnueabi/libvirt/1.3.5-r0/build/src/../../libvirt-1.3.5/src/util/virobject.c:153: undefined reference to `virAtomicLock'
| ../tools/nss/.libs/libnss_libvirt_impl.a(libvirt_nss_la-virobject.o): In function `virObjectNew':
| /buildarea2/kkang/builds/qemuarm-Aug03/bitbake_build/tmp/work/armv5e-wrs-linux-gnueabi/libvirt/1.3.5-r0/build/src/../../libvirt-1.3.5/src/util/virobject.c:205: undefined reference to `virAtomicLock'
| ../tools/nss/.libs/libnss_libvirt_impl.a(libvirt_nss_la-virobject.o): In function `virObjectUnref':
| /buildarea2/kkang/builds/qemuarm-Aug03/bitbake_build/tmp/work/armv5e-wrs-linux-gnueabi/libvirt/1.3.5-r0/build/src/../../libvirt-1.3.5/src/util/virobject.c:277: undefined reference to `virAtomicLock'
| ../tools/nss/.libs/libnss_libvirt_impl.a(libvirt_nss_la-virobject.o): In function `virObjectRef':
| /buildarea2/kkang/builds/qemuarm-Aug03/bitbake_build/tmp/work/armv5e-wrs-linux-gnueabi/libvirt/1.3.5-r0/build/src/../../libvirt-1.3.5/src/util/virobject.c:298: undefined reference to `virAtomicLock'
| collect2: error: ld returned 1 exit status

It is similar with:

http://libvirt.org/git/?p=libvirt.git;a=commit;h=12dc729Signed-off-by: NKai Kang <kai.kang@windriver.com>

src/Makefile.am

@@ -2997,6 +2997,8 @@ noinst_LTLIBRARIES += libvirt-nss.la
 libvirt_nss_la_SOURCES =		\
 		util/viralloc.c			\
 		util/viralloc.h			\
+		util/viratomic.c		\
+		util/viratomic.h		\
 		util/virbitmap.c		\
 		util/virbitmap.h		\
 		util/virbuffer.c		\