irc/bitchx: fix non-static inline functions

During an exp-run for llvm 13 (see bug 258209), it turned out that both
chinese/bitchx and irc/bitchx fail to build with clang 13 [1]:

...
cc -fstack-protector-strong -L/usr/lib -o BitchX alias.o alist.o array.o art.o banlist.o bot_link.o cdcc.o cdns.o chelp.o commands.o commands2.o compat.o cset.o ctcp.o dcc.o debug.o encrypt.o exec.o files.o flood.o fset.o functions.o funny.o glob.o hash.o hebrew.o help.o history.o hook.o if.o ignore.o input.o irc.o ircaux.o ircsig.o keys.o lastlog.o list.o log.o mail.o misc.o modules.o names.o network.o newio.o notice.o notify.o numbers.o output.o parse.o queue.o readlog.o reg.o screen.o server.o stack.o status.o struct.o tcl_public.o term.o timer.o translat.o user.o userlist.o vars.o who.o whowas.o window.o words.o -ldl -ltinfo -lssl -lcrypto -lm -lcrypt
ld: error: undefined symbol: operator

referenced by alias.c

alias.o:(zzlex)

cc: error: linker command failed with exit code 1 (use -v to see invocation)

This is because several functions in source/expr2.c are marked __inline,
without either static or extern keyword. The compiler then has to assume
the function is also externally available.

Fix this by marking the affected functions static.

PR: 258464
Approved by: fernape (maintainer)
MFH: 2021Q4