protocol/ArgParser: cast seek offset to SignedSongTime

"The issue is that ParseCommandArgSignedSongTime parses with
SongTime::FromS, not SignedSongTime::FromS, before casting back to a
SignedSongTime for the return. With x86 overflow rules this doesn't
matter, but on ARM the first cast turns negative values to zero."

Closes https://github.com/MusicPlayerDaemon/MPD/issues/757

src/protocol/ArgParser.cxx

@@ -175,5 +175,5 @@ SignedSongTime
 ParseCommandArgSignedSongTime(const char *s)
 {
 	auto value = ParseCommandArgFloat(s);
-	return SongTime::FromS(value);
+	return SignedSongTime::FromS(value);
 }