atomic: Remove cdefs.h includes from atomic_*.h

Avoid including cdefs.h in system headers.  Both headers now include
types.h, and we can assume that that pulls in cdefs.h (required for
__typeof usage in some of the atomic macro expansions).

No functional change intended.

Reviewed by:	imp, kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D38039

sys/sys/atomic_common.h

@@ -36,7 +36,6 @@
 #error do not include this header, use machine/atomic.h
 #endif
 
-#include <sys/cdefs.h>
 #include <sys/types.h>
 
 #define	__atomic_load_bool_relaxed(p)	(*(volatile _Bool *)(p))

sys/sys/atomic_san.h

@@ -43,7 +43,6 @@
 #error do not include this header, use machine/atomic.h
 #endif
 
-#include <sys/cdefs.h>
 #include <sys/types.h>
 
 #define	ATOMIC_SAN_FUNC_1(sp, op, name, type)				\