[gdb] Fix -Wstrict-null-sentinel warning (--with-iconv-bin)

[gdb] Fix -Wstrict-null-sentinel warning (--with-iconv-bin)

When using configure flag --with-iconv-bin=$(which iconv), we run into:
...
gdb/charset.c: In function 'void find_charset_names()':
gdb/charset.c:821:75: error: missing sentinel in function call [-Werror=format=]
     iconv_program = concat (iconv_dir.c_str(), SLASH_STRING, "iconv", NULL);
                                                                           ^
cc1plus: all warnings being treated as errors
...

Fix the warning.

Build and reg-tested on x86_64-linux.

2020-02-12  Lukas Durfina  <ldurfina@tachyum.com>
	    Tom de Vries  <tdevries@suse.de>

	* charset.c (find_charset_names): Cast concat NULL sentinel to char *.