build: fix missing include of ctype.h

[maxux]

Vanilla repository doesn't build on my Gentoo with GCC 12.

c++ -std=c++14 -Wall -Wextra -Ofast -DHAVE_PCAP=1 -lpcap -lpthread -lssl -lcrypto main.cpp -o tunnel
In file included from factory.cpp:2,
                 from main.cpp:1:
shared.cpp: In function ‘int parse_protocol_tag(char*)’:
shared.cpp:364:14: error: ‘tolower’ was not declared in this scope
  364 |         *c = tolower(*c);
      |              ^~~~~~~

This pull request add the missing include of ctype.h :)