MDEV-24131: unittest stacktrace-t fails to compile (OpenBSD)

This was because OpenBSD (and others) can be missing HAVE_BACKTRACE / HAVE_BACKTRACE_FD
which is the condition for my_safe_print_str to be defined.

Fixes:
/prj/tmp/work/cortexa57-poky-linux-musl/mariadb/10.5.8-r0/recipe-sysroot-native/usr/bin/aarch64-poky-linux-musl/../../libexec/aarch64-poky-linux-musl/gcc/aarch64-poky-linux-musl/10.2.0/ld.bfd: /usr/src/debug/mariadb/10.5.8-r0/mariadb-10.5.8/unittest/mysys/stacktrace-t.c:36: undefined reference to `my_safe_print_str'
Signed-off-by: Mingli Yu <mingli.yu@windriver.com>

unittest/mysys/stacktrace-t.c

@@ -29,6 +29,7 @@ void test_my_safe_print_str()
   memcpy(b_stack, "LEGAL", 6);
   memcpy(b_bss, "LEGAL", 6);
 
+#ifdef HAVE_STACKTRACE
 #ifndef __SANITIZE_ADDRESS__
   fprintf(stderr, "\n===== stack =====\n");
   my_safe_print_str(b_stack, 65535);
@@ -48,6 +49,7 @@ void test_my_safe_print_str()
   fprintf(stderr, "\n===== (const char*) 1 =====\n");
   my_safe_print_str((const char*)1, 5);
 #endif /*__SANITIZE_ADDRESS__*/
+#endif /*HAVE_STACKTRACE*/
 
   free(b_heap);