Set size of perf_attr_struct prior to getting pfm encoding

[jishminor]

Pull Request Description

Simple fix to ensure correct size of struct gets mem copied in pfm_lib_perf_event_encode in libpfm4.

Events from pfm4 which contain values in the config2 field of the perf_event_attr struct seem to not get copied in pfm_lib_perf_event_encode when opening up a PAPI event.

The general trace of this issue is:

1. ntv_evt->attr is zeroed out here
2. attr.size is 0 here, resulting in PERF_ATTR_SIZE_VER0 being used as the default size used to memcpy the perf_event_attr which the user gets back from the function here.
3. This results in additional fields required to open some events in perf such as config2 not being set when requested.