gsm: ensure completeness of osmo_bts_features_{descs,names}[]

It already happened several times [1][2] that new features were added
to enum osmo_bts_features, but the osmo_bts_features_{descs,names}[]
were left unchanged.  Let's add static_assert()s to prevent this.

Change-Id: I8e3b7d3996e9f3e16c6d4e0d1d406fa538d5e9be
Related: [1] f4f5d54ea2cb47a51aeaec2b5fb990664755900a
Related: [2] 18c6a8183f92915e77368ecffb1cbf7f555453a3
diff --git a/include/osmocom/gsm/bts_features.h b/include/osmocom/gsm/bts_features.h
index d6eb246..cf1db4a 100644
--- a/include/osmocom/gsm/bts_features.h
+++ b/include/osmocom/gsm/bts_features.h
@@ -7,7 +7,7 @@
 
 /* N. B: always add new features to the end of the list (right before _NUM_BTS_FEAT) to avoid breaking compatibility
    with BTS compiled against earlier version of this header. Also make sure that the description strings
-   osmo_bts_features_descs[] in gsm_data.c are also updated accordingly! */
+   osmo_bts_features_{descs,names}[] in bts_features.c are also updated accordingly! */
 enum osmo_bts_features {
 	BTS_FEAT_HSCSD,
 	BTS_FEAT_GPRS,