Fix C type safety issue in CVE-2023-30547 change

The types char ** and const char ** are distinct according to the
C language.  Future compilers will treat pointer type mismatches
as errors.

Submitted upstream: <https://github.com/zmanda/amanda/pull/242>

diff --git a/client-src/runtar.c b/client-src/runtar.c
index 454e9b9a9b96449b..4d2451dc84840fa9 100644
--- a/client-src/runtar.c
+++ b/client-src/runtar.c
@@ -54,7 +54,7 @@ main(
 {
 #ifdef GNUTAR
     int i;
-    char **j;
+    const char **j;
     char *e;
     char *dbf;
     char *cmdline;
@@ -255,7 +255,7 @@ check_whitelist(
     gchar* option)
 {
     bool result = TRUE;
-    char** i;
+    const char** i;
 
     for(i=whitelisted_args; *i; i++) {
         if (g_str_has_prefix(option, *i)) {