remove illegal `-f' ar option on Darwin

[JLHwung]

This PR fixes compilation error on macOS 10.12

ar crf src/convert_woff2ttf_fuzzer.a  src/font.o  src/glyph.o  src/normalize.o  src/table_tags.o  src/transform.o  src/woff2_dec.o  src/woff2_enc.o  src/woff2_common.o  src/woff2_out.o  src/variable_length.o brotli/enc/*.o brotli/dec/*.o src/convert_woff2ttf_fuzzer.o
/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/ar: illegal option -- f
usage:  ar -d [-TLsv] archive file ...
	ar -m [-TLsv] archive file ...
	ar -m [-abiTLsv] position archive file ...
	ar -p [-TLsv] archive [file ...]
	ar -q [-cTLsv] archive file ...
	ar -r [-cuTLsv] archive file ...
	ar -r [-abciuTLsv] position archive file ...
	ar -t [-TLsv] archive [file ...]
	ar -x [-ouTLsv] archive [file ...]
make: *** [convert_woff2ttf_fuzzer] Error 1

man ar suggests there is no -f option on ar.

This PR removes -f option to make it work on macOS. I am not sure if this option is vital on building process so any suggestions are welcome.

[dmitshur]

PR title says r, but it's actually f.

The corresponding issue is #73.

[JLHwung]

@shurcooL Good catch! corrected the PR title.

[rsheeter]

Thanks!

[JLHwung]

You are very welcome.