test: fix compilation warning
test: fix compilation warning

file_prefix_in_subdir is only used in Windows and OSX. Fixes the
following warning, which shows up with Clang:

test/test-fs-event.c:40:19: warning: unused variable
'file_prefix_in_subdir' [-Wunused-const-variable]
static const char file_prefix_in_subdir[] = "subdir";

PR-URL: https://github.com/libuv/libuv/pull/450
Reviewed-By: Saúl Ibarra Corretgé <8a7991f6950ea189abf159d6ec98d2fcc13dc5f6@gmail.com>

unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso,unknownbrackets/maxcso