package/libsrtp: bump to version 2.4.2

This is a patch release for version 2 & 2.4, fixing a regression
introduced in 2.4.1.

https://github.com/cisco/libsrtp/releases/tag/v2.4.2



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