Added additional instruction to 1_installation.rst: run sudo ldconfig

[adivh]

I added the instruction to run sudo ldconfig after installing srsRAN_4G from source.

On Ubuntu Desktop 22.04 LTS, not running sudo ldconfig will cause errors when executing srsue and srsenb:

$ sudo srsue
srsue: error while loading shared libraries: libsrsran_rf.so.0: cannot open shared object file: No such file or directory

$ sudo srsenb
srsenb: error while loading shared libraries: libsrsran_rf.so.0: cannot open shared object file: No such file or directory