linux: Fix missing __convert_scm_timestamps (BZ #28860)

Commit 948ce73b made recvmsg/recvmmsg to always call
__convert_scm_timestamps for 64 bit time_t symbol, so adjust it to
always build it for __TIMESIZE != 64.

It fixes build for architecture with 32 bit time_t support when
configured with minimum kernel of 5.1.