objclass: move cls_log into class_api.cc

Not sure why but this seems to resolve a linking problem when loading
classes:

2013-08-17 13:28:19.015776 7fb2bcffa700  0 _load_class could not open class /usr/lib/rados-classes/libcls_hello.so (dlopen failed): /usr/lib/rados-classes/libcls_hello.so: undefined symbol: cls_log
2013-08-17 13:28:19.015786 7fb2bcffa700 -1 osd.4 12 class hello open got (5) Input/output error

In any case, it's simpler.

Signed-off-by: Sage Weil <sage@inktank.com>