Fix C error reported by Oracle compiler.

Commit 66aaabe7 (branches 13 - 17 only) was not acceptable to the Oracle
Developer Studio compiler on build farm animal wrasse.  It accidentally
used a C++ style return statement to wrap a void function.  None of the
usual compilers complained, but it is right, that is not allowed in C.
Fix.

Reported-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/Z33vgfVgvOnbFLN9%40paquier.xyz

src/backend/storage/smgr/smgr.c

@@ -557,7 +557,7 @@ smgrtruncate(SMgrRelation reln, ForkNumber *forknum, int nforks,
 	for (int i = 0; i < nforks; ++i)
 		old_nblocks[i] = smgrnblocks(reln, forknum[i]);
 
-	return smgrtruncate2(reln, forknum, nforks, old_nblocks, nblocks);
+	smgrtruncate2(reln, forknum, nforks, old_nblocks, nblocks);
 }
 
 /*