atomic: mark header functions static

atomic: mark header functions static

When gcc atomic intrinsics are not available (such as on RHEL 5
with gcc 4.1.2), we were getting link errors due to multiple
definitions:

./.libs/libvirt_util.a(libvirt_util_la-virobject.o): In function `virAtomicIntXor':
/home/dummy/l,ibvirt/src/util/viratomoic.h:404: multiple definition of `virAtomicIntXor'
./.libs/libvirt_util.a(libvirt_util_la-viratomic.o):/home/dummy/libvirt/src/util/viratomic.h:404: first defined here

Solve this by conditionally marking the functions static (the
condition avoids falling foul of gcc warnings about unused
static function declarations).

* src/util/viratomic.h: When not using gcc intrinsics, use static
functions to avoid linker errors on duplicate functions.