BUILD: mux_fcgi: avoid aliasing of a const struct in traces

BUILD: mux_fcgi: avoid aliasing of a const struct in traces

fcgi_trace() declares fconn as a const and casts its mbuf array to
(struct buffer*), which rightfully upsets some older compilers. Better
just declare it as a writable variable and get rid of the cast. It's
harmless anyway. This has been there since 2.1 with commit 5c0f859c2
("MINOR: mux-fcgi/trace: Register a new trace source with its events")
and doens't need to be backported though it would not harm either.