Fix potential compile error "unused function"

__system_property_set is defined as a wrapper to
android::base::SetProperty if bionic is unavailable. That can cause
compiler error "unused function" if no setters exist. Fixing it with
adding [[maybe_unused]]

Bug: 156698358
Test: sysprop_test
Test: define a sysprop_library and link against it from cc_test_host
Change-Id: I1a2420793cc7b31d80031517113ca59a109d4a1c
2 files changed