missing: add getdents64() syscall wrapper

glibc 2.30 (Aug 2019) added a wrapper for getdents64(). For older
versions let's define our own.

(This syscall exists since Linux 2.4, hence should be safe to use for
us)

src/basic/missing_syscall.h

@@ -540,3 +540,19 @@ static inline int missing_move_mount(
 
 #  define move_mount missing_move_mount
 #endif
+
+/* ======================================================================= */
+
+#if !HAVE_GETDENTS64
+
+static inline ssize_t missing_getdents64(int fd, void *buffer, size_t length) {
+#  if defined __NR_getdents64 && __NR_getdents64 >= 0
+        return syscall(__NR_getdents64, fd, buffer, length);
+#  else
+        errno = ENOSYS;
+        return -1;
+#  endif
+}
+
+#  define getdents64 missing_getdents64
+#endif