Makefile: correct example fuzz build

Makefile: correct example fuzz build

The comment explaining how to build the fuzzers was broken in
927c77e7d4d ("Makefile: use FUZZ_CXXFLAGS for linking fuzzers",
2018-11-14).

When building fuzzers, all .c files must be compiled with coverage
tracing enabled. This is not possible when using only FUZZ_CXXFLAGS, as
that flag is only applied to the fuzzers themselves. Switching back to
CFLAGS fixes the issue.

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>