Prevent compilation of frontend-only files in src/common/ with backend

Any frontend-only file of src/common/ should include a protection to
prevent such code to be included in the backend compilation.
fe_memutils.c and restricted_token.c have been doing that, while
file_utils.c (since bf5bb2e8) and logging.c (since fc9a62af) forgot it.

Reviewed-by: Daniel Gustafsson
Discussion: https://postgr.es/m/20200625080757.GI130132@paquier.xyz

src/common/file_utils.c

@@ -12,6 +12,11 @@
  *
  *-------------------------------------------------------------------------
  */
+
+#ifndef FRONTEND
+#error "This file is not expected to be compiled for backend code"
+#endif
+
 #include "postgres_fe.h"
 
 #include <dirent.h>

src/common/logging.c

@@ -7,6 +7,11 @@
  *
  *-------------------------------------------------------------------------
  */
+
+#ifndef FRONTEND
+#error "This file is not expected to be compiled for backend code"
+#endif
+
 #include "postgres_fe.h"
 
 #include <unistd.h>