gzio.c: Remove unnecessary usage of the OF() macro

Daniel Goertzen reported that commit 8a147a7365 broke building
of Erlang/OTP on Gentoo Linux because the macro OF() was missing.
Apparently, on Gentoo the OF() macro in zconf.h has been renamed
to _Z_OF() (to avoid polluting the global namespace).

Don't use the OF() macro in gzio.c since it no longer serves any
useful purpose (it provided compatibility with pre-ANSI/ISO C
compilers, but the rest of Erlang/OTP requires an ANSI/ISO C
compiler anyway).