seemingly useless #include recently added

Hello.

While working on a patch, I noticed that a rcent commit (d4e71df6d75)
added an apparently unnecessary inclusion of guc.h in smgr.h.

The only change made by the commit to the file is the added #include
directive, which doesn't seem to be functioning, and the build
actually suceeds without it.  Moreover, it brings in some
server-related stuff when I incluce smgr.h in storage_xlog.h, causing
compilation issues for pg_rewind.

Should we remove it? Please find the attached patch.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center