libcurl: security bump to 7.52.1

libcurl: security bump to 7.52.1

Fixes CVE-2016-9594 - Unitilized random

Libcurl's (new) internal function that returns a good 32bit random value was
implemented poorly and overwrote the pointer instead of writing the value
into the buffer the pointer pointed to.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>