libctf: mark various args as unused in the !HAVE_MMAP case

Tested on x86_64-pc-linux-gnu, x86_64-unknown-freebsd12.0,
sparc-sun-solaris2.11, i686-pc-cygwin, i686-w64-mingw32.

libctf/
	* ctf-archive.c (arc_mmap_header): Mark fd as potentially unused.
	* ctf-subr.c (ctf_data_protect): Mark both args as potentially unused.