package/libcurl: bump version to 7.68.0

package/libcurl: bump version to 7.68.0

Notice that 7.68.0 includes a Windows-only security fix:

- CVE-2019-15601: SMB access smuggling via FILE URL on Windows
  https://curl.haxx.se/docs/CVE-2019-15601.html

So not applicable to Buildroot.

Update the license hash for a copyright year update:

-Copyright (c) 1996 - 2019, Daniel Stenberg, <daniel@haxx.se>, and many
+Copyright (c) 1996 - 2020, Daniel Stenberg, <daniel@haxx.se>, and many

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>