monitor: Fix const qualifier build errors with recent glibc

A recent change in glibc 2.42.9000 [1] changes the return type of
strchr() and other string functions to be 'const char *' when the
input is a 'const char *'. This breaks the build in :

../monitor/hmp.c:589:7: error: assignment discards ‘const’ qualifier from pointer target type [-Werror=discarded-qualifiers]
  589 |     p = strchr(type, ':');
      |       ^

Fix this by changing the type of the variables that store the result
of these functions to 'const char *'.

[1] https://sourceware.org/git/?p=glibc.git;a=commit;h=cd748a63ab1a7ae846175c532a3daab341c62690

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/qemu-devel/20251215101937.281722-4-clg@redhat.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>

monitor/hmp.c

@@ -577,10 +577,11 @@ static const char *get_command_name(const char *cmdline,
  * Read key of 'type' into 'key' and return the current
  * 'type' pointer.
  */
-static char *key_get_info(const char *type, char **key)
+static const char *key_get_info(const char *type, char **key)
 {
     size_t len;
-    char *p, *str;
+    const char *p;
+    char *str;
 
     if (*type == ',') {
         type++;