virt-login-shell: also build virAtomic.h

virt-login-shell: also build virAtomic.h

Needed for architectures that don't use gcc atomic ops but pthread. This
fixes the armel build that otherwise breaks like:

 CCLD     virt-login-shell
 ../src/.libs/libvirt-setuid-rpc-client.a(libvirt_setuid_rpc_client_la-virobject.o): In function `virClassNew':
 /«PKGBUILDDIR»/debian/build/src/../../../src/util/virobject.c:150: undefined reference to `virAtomicLock'
 ../src/.libs/libvirt-setuid-rpc-client.a(libvirt_setuid_rpc_client_la-virobject.o): In function `virObjectNew':
 /«PKGBUILDDIR»/debian/build/src/../../../src/util/virobject.c:202: undefined reference to `virAtomicLock'
 ../src/.libs/libvirt-setuid-rpc-client.a(libvirt_setuid_rpc_client_la-virobject.o): In function `virObjectUnref':
 /«PKGBUILDDIR»/debian/build/src/../../../src/util/virobject.c:274: undefined reference to `virAtomicLock'
 ../src/.libs/libvirt-setuid-rpc-client.a(libvirt_setuid_rpc_client_la-virobject.o): In function `virObjectRef':
 /«PKGBUILDDIR»/debian/build/src/../../../src/util/virobject.c:295: undefined reference to `virAtomicLock'
 collect2: error: ld returned 1 exit status

See https://buildd.debian.org/status/fetch.php?pkg=libvirt&arch=armel&ver=1.1.4-1&stamp=1383588268