Fix namespacing errors

Such as:
src/fs_base_utimensat.hpp:34:12: error: no member named 'utimensat' in the global namespace

src/fs_base_fsync.hpp

@@ -33,6 +33,6 @@ namespace fs
   int
   fdatasync(const int fd)
   {
-    return ::fdatasync(fd);
+    return fs::fdatasync(fd);
   }
 }

src/fs_base_utimensat.hpp

@@ -31,7 +31,7 @@ namespace fs
             const struct timespec  times[2],
             const int              flags)
   {
-    return ::utimensat(dirfd,path.c_str(),times,flags);
+    return fs::utimensat(dirfd,path.c_str(),times,flags);
   }
 
   static