Fix "make install-strip" failure to install gdb-add-index.sh
    
    gdb/ChangeLog:
    2018-09-10  Eli Zaretskii  <eliz@gnu.org>
    
    	* Makefile.in (transformed_name): Use INSTALL_SCRIPT instead of
    	INSTALL_PROGRAM to install gdb-add-index.sh.  Don't append
    	$(EXEEXT) to the script, as it is not a program.