Add missing chmap positions

These four positions were missing in the rust definition of the enum. No
idea why. However, that made it impossible to construct any standard
surround channel map (which need rear-right (4.0 onward),
LFE (2.1 onward), front-center (5.0 onward), or side-left (7.1 onward)).

src/chmap.rs

@@ -14,14 +14,18 @@ alsa_enum!(
 
 alsa_enum!(
     /// [SND_CHMAP_xxx](http://www.alsa-project.org/alsa-doc/alsa-lib/group___p_c_m.html) constants
-    ChmapPosition, ALL_CHMAP_POSITIONS[33],
+    ChmapPosition, ALL_CHMAP_POSITIONS[37],
 
     Unknown = SND_CHMAP_UNKNOWN,
     NA = SND_CHMAP_NA,
     Mono = SND_CHMAP_MONO,
     FL = SND_CHMAP_FL,
     FR = SND_CHMAP_FR,
     RL = SND_CHMAP_RL,
+    RR = SND_CHMAP_RR,
+    FC = SND_CHMAP_FC,
+    LFE = SND_CHMAP_LFE,
+    SL = SND_CHMAP_SL,
     SR = SND_CHMAP_SR,
     RC = SND_CHMAP_RC,
     FLC = SND_CHMAP_FLC,