libutempter: Fix compile error

| iface.c: In function 'utempter_add_record':
| <command-line>:0:12: error: expected expression before '/' token
| iface.c:46:35: note: in expansion of macro 'LIBEXECDIR'
|  #define UTEMPTER_DEFAULT_PATHNAME LIBEXECDIR "/utempter/utempter"

Signed-off-by: Qian Lei <qianl.fnst@cn.fujitsu.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>