Fix ppoll() breakage by fortify-headers
The fortify-headers/poll.h definitions incorrectly forward the ppoll() function: on some archs (armv7 for instance), musl renames the symbol to accommodate for time_t size, and fortify-headers misses that.
This causes ppoll() to break on all time32 systems.
This commit axes the ppoll() redefinition. Fortifying ppoll() is of very little benefit anyway.