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/gsm/protocol/gsm_23_041.h b/include/osmocom/gsm/protocol/gsm_23_041.h
index 564946c..2a2b006 100644
--- a/include/osmocom/gsm/protocol/gsm_23_041.h
+++ b/include/osmocom/gsm/protocol/gsm_23_041.h
@@ -22,7 +22,7 @@
 		uint8_t num_pages:4,
 			page_nr: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 page_nr:4, num_pages:4;
 #endif
 	} page_param;