basebackup_to_shell: Add missing MarkGUCPrefixReserved()

Oversight in c6306db24, as per a requirement from 88103567.  All the
other modules in the tree, be they in contrib/ or src/test/modules/,
already do that.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACUy7q_KwSMda+2SHPSWep32tNUM8cXGRS3=-Vfodo9OUg@mail.gmail.com

contrib/basebackup_to_shell/basebackup_to_shell.c

@@ -86,6 +86,8 @@ _PG_init(void)
 							   0,
 							   NULL, NULL, NULL);
 
+	MarkGUCPrefixReserved("basebackup_to_shell");
+
 	BaseBackupAddTarget("shell", shell_check_detail, shell_get_sink);
 }