buildroot

libsrtp: fix SHA1 functions conflicts with downstream packages

Our current version of libsrtp is preventing linphone to build causing
an error like this one:

configure:12952: error: This libsrtp version exports symbols conflicting
with other libraries. Please use the one from
git://git.linphone.org/srtp.git

Backporting an upstream patch to rename the SHA1 functions to avoid
conflicts with downstream packages:

  https://github.com/cisco/libsrtp/commit/c270245a94ae9a007202754eb8f7ce9e48f97007

Fixes:

  http://autobuild.buildroot.net/results/773/7733ee4472834d26cf6bdd4ca557ac19f97290f8/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>