Use NAME_MAX instead of MAXNAMLEN

NAME_MAX is POSIX, and available in all C libraries, generally in
<limits.h>, while MAXNAMLEN is BSD-specific, and only available in
musl in <sys/param.h>. So let's use NAME_MAX instead of MAXNAMLEN.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: b/src/index_dir.c
===================================================================
--- a/src/index_dir.c
+++ b/src/index_dir.c
@@ -29,7 +29,7 @@
 #include <fcntl.h>
 #include "compat.h"
 
-#define MAX_FILE_LENGTH                         MAXNAMLEN
+#define MAX_FILE_LENGTH                         NAME_MAX
 #define MAX_PATH_LENGTH                         PATH_MAX
 
 #define INT_TO_HEX(x) \