unigbrk/uc-grapheme-breaks tests: Fix gcc -Wunused-function warning

[Bruno Haible]

This warning exists for a pretty long time as well:

unigbrk/test-uc-grapheme-breaks.c:31:1: warning: 
'graphemebreakproperty_to_string' defined but not used [-Wunused-function]


2023-09-04  Bruno Haible  <br...@clisp.org>

        unigbrk/uc-grapheme-breaks tests: Fix gcc -Wunused-function warning.
        * tests/unigbrk/test-uc-grapheme-breaks.c
        (graphemebreakproperty_to_string): Mark as possibly unused.

diff --git a/tests/unigbrk/test-uc-grapheme-breaks.c 
b/tests/unigbrk/test-uc-grapheme-breaks.c
index b898dc1c9f..6310d750c9 100644
--- a/tests/unigbrk/test-uc-grapheme-breaks.c
+++ b/tests/unigbrk/test-uc-grapheme-breaks.c
@@ -27,7 +27,7 @@
 
 #include "macros.h"
 
-static const char *
+_GL_UNUSED static const char *
 graphemebreakproperty_to_string (int gbp)
 {
   printf ("%d\n", gbp);