FYI, avoid new GCC11 warning

[Jim Meyering]

FYI, I've just pushed this:

    maint: avoid new sort.c warning from upcoming GCC11

    gcc version 11.0.0 20201025 (experimental) warns that
    src/sort.c:1655:1: warning: function might be candidate for attribute \
      'pure' if it is known to return normally [-Wsuggest-attribute=pure]
    * src/sort.c (limfield): Mark as pure.

diff --git a/src/sort.c b/src/sort.c
index 0163ba18a..8671ea767 100644
--- a/src/sort.c
+++ b/src/sort.c
@@ -1651,7 +1651,7 @@ begfield (struct line const *line, struct
keyfield const *key)
 /* Return the limit of (a pointer to the first character after) the field
    in LINE specified by KEY. */

-static char *
+static char * _GL_ATTRIBUTE_PURE
 limfield (struct line const *line, struct keyfield const *key)
 {
   char *ptr = line->text, *lim = ptr + line->length - 1;