packaging: rhel8 needs json-cpp and up-to-date gnutls

add BuildRequires jsoncpp-devel to the spec files, otherwise
buliding RPM packages for RHEL 8 fails with this message

contrib/x86_64-redhat-linux checking for Jsoncpp... yes
checking for ZLIB... yes checking for PJPROJECT... no
configure: error: Missing pjproject files error: Bad exit status ...

Add '.gnutls' after 'make -j4 V=1 .ffmpeg' in the spec file
if we don't compile GnuTLS Jami is unable to establish p2p communication

Error message in dring when using RHEL8's GnuTLS version 3.6.8
[TLS] handshake failed: An error has been detected in the library and cannot
continue operations.

Change-Id: Ic8c500eaa286d66f251588f8052267d0bfc3240f