LU-14093 llapi: remove ignored qualifier

LU-14093 llapi: remove ignored qualifier

Fixes the following warning on newer gcc with -Wextra:
.../include/lustre/lustreapi.h:1000:1: error: type qualifiers ignored on function return type [-Werror=ignored-qualifiers]
 1000 | const __u16 llapi_layout_string_flags(char *string);
      | ^~~~~

As the parameter is ignored, this should make no code difference

Test-parameters: trivial

Change-Id: I049166bbc586007cdecc93225d508693607ef04e
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-on: https://review.whamcloud.com/43712
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>