Meson build scripts will only include qemu-fuzz-TARGET rules if configured
with --enable-fuzzing, and that takes care of adding -fsanitize=fuzzer.
Therefore we can just specify the configure option and stop modifying
the CFLAGS and CONFIG_FUZZ options in the "make" invocation.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>