libpjsip: fix static build failures due to name clash

Several packages have a similar md5.c file, and each has a function
named byteReverse(). This generates link errors when building
statically ("multiple definition of `byteReverse'").

Fix by applying a patch from upstream:
  https://trac.pjsip.org/repos/changeset/5688

Fixes:
  http://autobuild.buildroot.org/results/5d7/5d72e0f8517a555399978d5a0e9f7efd0a278189/
  http://autobuild.buildroot.org/results/c47/c47ccbb9b40011cf0d79c7040bed061ddefd9629/
  http://autobuild.buildroot.org/results/419/419ab2c0e034cc68991281c51caa8271b0fadbab/



Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Adam Duskett <aduskett@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>