Include codec.h file from codec C files

failure to do so would prevent the compiler from catching
inconsistencies between declaration and definition.

Change-Id: I88617c97adcff328292e501fd5d254eeae96a660
diff --git a/src/codec/gsm660.c b/src/codec/gsm660.c
index c044a2a..6c9215f 100644
--- a/src/codec/gsm660.c
+++ b/src/codec/gsm660.c
@@ -22,6 +22,7 @@
  */
 
 #include <stdint.h>
+#include <osmocom/codec/codec.h>
 
 /* GSM EFR - subjective importance bit ordering */
 	/* This array encodes GSM 05.03 Table 6.