rgw: do not include "expat.h" in heade file

rgw: do not include "expat.h" in heade file

Building Ceph without the Expat development library currently results in
the following build error:

~/ceph> ./configure --without-fuse --without-tcmalloc --without-radosgw
...
~/ceph> make
...
CXX ceph_dencoder-rgw_dencoder.o
In file included from rgw/rgw_dencoder.cc:6:0:
rgw/rgw_acl_s3.h:9:19: fatal error: expat.h: No such file or directory

The ceph-dencoder binary is built with rgw_dencoder.cc, which includes
rgw_acl_s3.h, which in turn includes expat.h. However, configure.ac
currently only checks for Expat as part of radosgw.

but we don't need expat.h for testing the encoding of rgw structs, so
move "#include <expat.h>" into the source file to avoid this dependency.
and as a side effect, this also speed up the compilation.

Signed-off-by: Kefu Chai <kchai@redhat.com>