devel/elfutils: fix future build failure w.r.t. stdio _unlocked

Include <stdio.h> early so that the stdio.h function declarations come
first, then redefine fputs/fwrite/fread_unlocked to the locked versions.
fputc_unlocked will be a macro similar to putc_unlocked, so just hide that
one behind an #ifndef since it will be provided as a macro by <stdio.h>. The
other three will be using the technically-incorrect locked variants until
all supported releases provide the needed functions, at which point we can
switch them all at once.

No PORTREVISION bump as this is just a preemptive build fix that doesn't
change the output at all.

QA:

• portlint not ran, no change outside of a single patch
• testport (-CURRENT, amd64; 12.1-RELEASE, amd64; -CURRENT+patch, amd64)

PR: 243546
Reviewed by: koobs (ports)
Approved by: koobs (ports), cem (maintainer)
MFH: 2020Q1 (implicit, build fix)