compel: print errors to stderr

After seeing a bunch of silent build failures, like this:

>   GEN      criu/pie/parasite-native-blob.h
> criu/pie/Makefile:121: recipe for target 'criu/pie/parasite-native-blob.h' failed

I finally took a look at why are they silent, only to discover that
compel prints errors to stdout, and of course its stdout is silenced
in criu/pie/Makefile (unless you run make with V=1, in which case
it prints tons and tons of very useful information). I am so shocked
by this evil plan!

Anyway, let's print errors to stderr like all sane programs do.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

compel/src/main.c

@@ -106,8 +106,15 @@ err:
 
 static void cli_log(unsigned int lvl, const char *fmt, va_list parms)
 {
-	if (!pr_quelled(lvl))
-		vprintf(fmt, parms);
+	FILE *f = stdout;
+
+	if (pr_quelled(lvl))
+		return;
+
+	if ((lvl == LOG_ERROR) || (lvl == LOG_WARN))
+		f = stderr;
+
+	vfprintf(f, fmt, parms);
 }
 
 static int usage(int rc) {