package/exempi: security bump to version 2.6.1

Extract from NEWS:
- Changes in v2021.08
  - Security Fixes
- Changes in v2021.07
  - Security Fixes

https://gitlab.freedesktop.org/libopenraw/exempi/-/blob/2.6.1/NEWS



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>