sample: fs: Fix a compile error flagged by llvm

printk format specifier used for size_t (%lu) was not compatible
with llvm because it sees size_t as unsigned int. Using %zu
makes it portable acroos compilers.

Change-Id: If0e732fbaada5f50a975ec912b8147d6b252a2de
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>

tests/filesystem/fat_fs/src/main.c

@@ -431,7 +431,7 @@ static int list_dir(const char *path)
 		if (entry.type == DIR_ENTRY_DIR) {
 			printk("[DIR ] %s\n", entry.name);
 		} else {
-			printk("[FILE] %s (size = %lu)\n",
+			printk("[FILE] %s (size = %zu)\n",
 				entry.name, entry.size);
 		}
 	}