lavu: use address-of operator checking clock_gettime

lavu: use address-of operator checking clock_gettime

When targeting a recent enough macOS/iOS version that has clock_gettime
it won't be a weak symbol, in which case clang warns for this check
as it's always true:

  warning: address of function 'clock_gettime' will always
  evaluate to 'true'

This warning is silenced by using the address-of operator to make
the intent explicit.