lang/gcc48: Set EXPIRATION_DATE

Commit 2c8ea965a6490057e4d4bb3ee5b46045352f55f0 has set an expiration
date for sysutils/uefi-edk2-bhyve-csm, which is the last port dependent
on lang/gcc48 with default options.

Hence, finally set an expiration date to the unsupported upstream
lang/gcc48 port. Expiration date is set one month after
the expiration of sysutils/uefi-edk2-bhyve-csm.

PR: 269646