util/probe: Save strrchr() ret val to const data ptr

With glibc-2.43 implementing the C23 standard, strrchr() now returns
"const char *" when its first argument is "const char *".

The fix is update all pointers receiving strrchr()'s return value so
that they are now "const char *" instead of "char *".

Signed-off-by: Nicholas Vinson <nvinson234@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/probe.c

@@ -70,7 +70,7 @@ char *
 grub_util_guess_bios_drive (const char *orig_path)
 {
   char *canon;
-  char *ptr;
+  const char *ptr;
   canon = grub_canonicalize_file_name (orig_path);
   if (!canon)
     return NULL;
@@ -99,7 +99,7 @@ char *
 grub_util_guess_efi_drive (const char *orig_path)
 {
   char *canon;
-  char *ptr;
+  const char *ptr;
   canon = grub_canonicalize_file_name (orig_path);
   if (!canon)
     return NULL;
@@ -128,7 +128,7 @@ char *
 grub_util_guess_baremetal_drive (const char *orig_path)
 {
   char *canon;
-  char *ptr;
+  const char *ptr;
   canon = grub_canonicalize_file_name (orig_path);
   if (!canon)
     return NULL;