devel/pkgconf: Update MASTER_SITES

Update to new URL as announced by upstream and while at it
remove stale mirror

References:
https://github.com/pkgconf/pkgconf/commit/437c2a3218bfcb1cae7fa38a4ccd0cb29575ff07
https://git.alpinelinux.org/aports/commit/?id=75a42157eee297c14726eb622d5ce50d99fc27ba

Approved by: portmgr (blanket)