helpers_fileio.c: fix fsync() check.

Both the _POSIX_FSYNC macro and the fsync() declaration are in
<unistd.h>. The current code works by accident on glibc courtesy of
transitive includes, but is broken in Android currently (mistakenly
assuming there is no fsync() because _POSIX_FSYNC isn't defined, merely
because <unistd.h> hasn't been included), and won't compile with Android's
C23 addition of call_once() to <stdlib.h> because _POSIX_FSYNC comes in
transitively via <limits.h> but without <unistd.h> for the fsync()
declaration.

Fix all of this by just including the right POSIX header file for both
the feature macro and the function declaration.

Change-Id: Ifd6eeba56cffabb6a37880c78ea21de4705308b5
Signed-off-by: Elliott Hughes <enh@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/90015
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

helpers_fileio.c

@@ -10,6 +10,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <errno.h>
+#include <unistd.h>
 
 #ifndef __LIBPAYLOAD__
 #include <fcntl.h>