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/src/gb/frame_relay.c b/src/gb/frame_relay.c
index 2c252fd..e973b91 100644
--- a/src/gb/frame_relay.c
+++ b/src/gb/frame_relay.c
@@ -138,7 +138,7 @@
 		ext2:1;
 
 #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 ext0:1, spare:1, dlci_msb:6;
 	uint8_t ext1:1, dlci_lsb:4, space1:3;
 	uint8_t ext2:1, spare2:3, new:1, delete:1, active:1, reserved:1;