parse-options: fix clang opterror() -Wunused-value warning

a469a1019352b8ef (silence some -Wuninitialized false positives;
2012-12-15) triggered "unused value" warnings when the return value of
opterror() and several other error-related functions was not used.
5ded807f7c0be10e (fix clang -Wunused-value warnings for error functions;
2013-01-16) applied a fix by adding #if !defined(__clang__) in cache.h
and git-compat-util.h, but misspelled it as #if !defined(clang) in
parse-options.h. Fix this.

This mistake went unnoticed because existing callers of opterror()
utilize its return value.  1158826394e162c5 (parse-options: add
OPT_CMDMODE(); 2013-07-30), however, adds a new invocation of opterror()
which ignores the return value, thus triggering the "unused value"
warning.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>