perforce.py: fix the perforce fetcher

The bb.process.run() will return one tuple, e.g:

p4file = ('strA\nStrB\nstrC\n'), then there will be an iteration on p4file:

for i in p4file:
	[snip]

The i will be 's t r A ...', this is incorrect. use splitlines() to fix
the problem.

[YOCTO #3619]

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>