Fix typo endianess -> endianness

Rename contrib/struct_endianess.py to contrib/struct_endianness.py, and
fix the typo everywhere. This is in preparation to call the script in
CI on all repositories.

Related: OS#5884
Change-Id: Idc4af9098ba1de26243464c772d6ea8be330646a
diff --git a/include/osmocom/gprs/protocol/gsm_08_18.h b/include/osmocom/gprs/protocol/gsm_08_18.h
index d6f0fe6..1152eb6 100644
--- a/include/osmocom/gprs/protocol/gsm_08_18.h
+++ b/include/osmocom/gprs/protocol/gsm_08_18.h
@@ -367,7 +367,7 @@
 			pdu_type_ext:3,
 			reserved:4;
 #elif OSMO_IS_BIG_ENDIAN
-/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianess.py) */
+/* auto-generated from the little endian part above (libosmocore/contrib/struct_endianness.py) */
 		uint8_t reserved:4, pdu_type_ext:3, ack_requested:1;
 #endif
 } __attribute__ ((packed));