redox: add FILENAME_MAX constant

https://github.com/redox-os/relibc/blob/381772ec81fced6d055f0f0ba6fa92c7f3256fbb/src/header/stdio/constants.rs#L8

(backport <#5530>)
(cherry picked from commit d3ec7b3)

Author: xtqqczze

libc-test/semver/redox.txt

@@ -101,6 +101,7 @@ EUCLEAN
 EUNATCH
 EUSERS
 EXFULL
+FILENAME_MAX
 FIONREAD
 F_DUPFD_CLOEXEC
 IMAXBEL

src/unix/redox/mod.rs

@@ -1190,6 +1190,7 @@ pub const X_OK: c_int = 1;
 
 // stdio.h
 pub const BUFSIZ: c_uint = 1024;
+pub const FILENAME_MAX: c_int = 4096;
 pub const _IOFBF: c_int = 0;
 pub const _IOLBF: c_int = 1;
 pub const _IONBF: c_int = 2;