patch 8.1.0268: file type checking has too many #ifdef

Problem:    File type checking has too many #ifdef.
Solution:   Always define the S_IF macros. (Ken Takata, closes #3306)
diff --git a/src/buffer.c b/src/buffer.c
index 32d1eb4..6405372 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -204,13 +204,8 @@
 #endif
 #ifdef UNIX
 	perm = mch_getperm(curbuf->b_ffname);
-	if (perm >= 0 && (0
-# ifdef S_ISFIFO
-		      || S_ISFIFO(perm)
-# endif
-# ifdef S_ISSOCK
+	if (perm >= 0 && (S_ISFIFO(perm)
 		      || S_ISSOCK(perm)
-# endif
 # ifdef OPEN_CHR_FILES
 		      || (S_ISCHR(perm) && is_dev_fd_file(curbuf->b_ffname))
 # endif