harfbuzz recipe build fail

### Logs

```

make[4]: Entering directory '/home/bobf/ex/regression/.buildozer/android/platform/build-arm64-v8a/build/other_builds/harfbuzz/arm64-v8a__ndk_target_21/harfbuzz/src'
make[4]: warning: -j12 forced in submake: resetting jobserver mode.
GEN libharfbuzz.la
GEN harfbuzz.pc
GEN harfbuzz-subset.pc
CXX libharfbuzz_subset_la-hb-subset-cff1.lo
CXXLD main
CXXLD test
CXXLD test-buffer-serialize
CXXLD test-ot-meta
CXXLD test-ot-name
CXXLD test-gpos-size-params
CXXLD test-gsub-would-substitute
hb-subset-cff1.cc:472:33: error: variable 'supp_size' set but not used [-Werror,-Wunused-but-set-variable]
unsigned int size0, size1, supp_size;
^
1 error generated.
```

Start with the HarfBuzz recipe build for the Android arm64-v8a target and inspect the compiler diagnostic at hb-subset-cff1.cc:472. Reproduce the logged build failure and determine what the recipe or build configuration must change; done means the HarfBuzz recipe builds without this warning being treated as an error.

Scritto dal modello di indicizzazione a partire dal testo della issue.