Add clang as a requirement
Add clang as a requirement

Clang was also required to run make.
```
ebiken@lab:~/github/slankdev/cuishark$ make
CXX main.o
make: clang++: Command not found
make: *** [main.o] Error 127
ebiken@lab:~/github/slankdev/cuishark$ sudo apt install clang
ebiken@lab:~/github/slankdev/cuishark$ make
CXX main.o
CXX TuiFrontend.o
CXX TextPane.o
CXX PacketListPane.o
CXX ToggleListPane.o
CXX protocol.o
LD cuishark
ebiken@lab:~/github/slankdev/cuishark$
```
slankdev/cuishark,slankdev/cuishark