BUG/MINOR: haproxy: Free proxy->unique_id_header during deinit

BUG/MINOR: haproxy: Free proxy->unique_id_header during deinit

Given the following example configuration:

    frontend foo
    	mode http
    	bind *:8080
    	unique-id-header x

Running a configuration check with valgrind reports:

    ==17621== 2 bytes in 1 blocks are definitely lost in loss record 1 of 341
    ==17621==    at 0x4C2DB8F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==17621==    by 0x649E489: strdup (strdup.c:42)
    ==17621==    by 0x4A87F1: cfg_parse_listen (cfgparse-listen.c:2747)
    ==17621==    by 0x494C59: readcfgfile (cfgparse.c:2049)
    ==17621==    by 0x545095: init (haproxy.c:2029)
    ==17621==    by 0x421E42: main (haproxy.c:3167)

After this patch is applied the leak is gone as expected.

This is a very minor leak that can only be observed if deinit() is called,
shortly before the OS will free all memory of the process anyway. No
backport needed.