fix: add missing n_cobs.c source file needed for binary examples

The n_cobs.c file is needed when building binary examples. Without it, the
build fails due to missing _cobs* symbol references in n_helpers.c

Author: youssifsaeed1

notecard/CMakeLists.txt

@@ -5,6 +5,7 @@ zephyr_library_sources(
 	../note-c/n_b64.c
 	../note-c/n_cjson.c
 	../note-c/n_cjson_helpers.c
+	../note-c/n_cobs.c
 	../note-c/n_const.c
 	../note-c/n_ftoa.c
 	../note-c/n_helpers.c