libtalloc: fix SRC_URI

It fails to fetch source of libtalloc:

| ERROR: Fetcher failure: Fetch command failed with exit code 8, output:
| https://www.samba.org/ftp/libtalloc/talloc-2.1.3.tar.gz

:
| 2015-12-21 10:22:09 ERROR 404: Not Found.

Fix it by replace ${BPN} with original package name talloc.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>