core: Move OSMO_DEPRECATED to new core/defs.h file

Add a new core/defs.h file for definitions that can be used from
within header files without including prototypes and extern
declarations. It's primarily meant for macro definitions and basic
typedefs.

Move the definition of OSMO_DEPRECATED there and use compiler version
info to avoid compiler errors.
diff --git a/include/osmocom/gsm/gsm_utils.h b/include/osmocom/gsm/gsm_utils.h
index 1c55dfb..f412e3e 100644
--- a/include/osmocom/gsm/gsm_utils.h
+++ b/include/osmocom/gsm/gsm_utils.h
@@ -28,7 +28,7 @@
 #include <stddef.h>
 #include <stdint.h>
 
-#include <osmocom/gsm/gsm_utils.h>
+#include <osmocom/core/defs.h>
 
 #define ADD_MODULO(sum, delta, modulo) do {	\
 	if ((sum += delta) >= modulo)		\
@@ -37,18 +37,6 @@
 
 #define GSM_MAX_FN	(26*51*2048)
 
-/*! \brief Set the deprecated attribute with a message.
- *
- * \todo Move this to a global header utility file.
- * \todo Check for compiler version to selectivly enable the message.
- */
-#if 0
-#define OSMO_DEPRECATED(text)  __attribute__((deprecated(text)))
-#else
-#define OSMO_DEPRECATED(text)  __attribute__((__deprecated__))
-#endif
-
-
 struct gsm_time {
 	uint32_t	fn;	/* FN count */
 	uint16_t	t1;	/* FN div (26*51) */