Don't rely on sys/_types.h including sys/cdefs.h

These headers relied in __BEGIN_DECS/__END_DECLS being defined when
sys/_types.h was included, but there's not a requirement that this be
the case.

Reviewed by:	imp
Exp-run by:	antoine (PR 286274)
Pull Request:	https://github.com/freebsd/freebsd-src/pull/1595

contrib/libexecinfo/execinfo.h

@@ -32,6 +32,8 @@
 #ifndef _EXECINFO_H_
 #define _EXECINFO_H_
 
+#include <sys/cdefs.h>
+
 #include <stddef.h>
 
 __BEGIN_DECLS

include/dlfcn.h

@@ -32,6 +32,7 @@
 #ifndef _DLFCN_H_
 #define	_DLFCN_H_
 
+#include <sys/cdefs.h>
 #include <sys/_types.h>
 
 /*

lib/msun/src/math.h

@@ -15,6 +15,7 @@
 #ifndef _MATH_H_
 #define	_MATH_H_
 
+#include <sys/cdefs.h>
 #include <sys/_types.h>
 #include <machine/_limits.h>
 

sys/sys/procdesc.h

@@ -108,6 +108,7 @@ int	 procdesc_falloc(struct thread *, struct file **, int *, int,
 
 #else /* !_KERNEL */
 
+#include <sys/cdefs.h>
 #include <sys/_types.h>
 
 #ifndef _PID_T_DECLARED