Update gettingstarted.rst by adding usage about compilation through c…

[foreveruzi]

we support configure compilation for OpenHarmony projects, so we'd like to add usage in the gettingstarted.rst file.

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[plusvic]

There's nothing specific to YARA in this PR. Its all about how to configure the tooling for building a generic C/++ project on OpenHarmony. In my opinion these instructions should belong to the OpenHarmony documentation itself, and not being replicated in every project that has the potential of being built in OpenHarmony.