Revert "use __dead macro from sys/cdefs.h"

This partially reverts commit a7577667d6.
The musl C library does not use an internal cdefs.h header. As a result,
the definitions in sys/cdefs.h have no effect. This breads the build as
follows:
In file included from adjfreq_linux.c:23:0:
../src/ntpd.h:448:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’
__dead void fatal(const char *, ...)
^~~~
../src/ntpd.h:450:8: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘void’
__dead void fatalx(const char *, ...)
^~~~
Refer the related musl FAQ about cdefs.h:
http://wiki.musl-libc.org/wiki/FAQ#Q:_I.27m_trying_to_compile_something_against_musl_and_I_get_error_messages_about_sys.2Fcdefs.h
Keep the compiler attribute definition.
Remove also from noinst_HEADERS.

include/Makefile.am

@@ -1,6 +1,5 @@
 noinst_HEADERS =
 noinst_HEADERS += machine/endian.h
-noinst_HEADERS += sys/cdefs.h
 noinst_HEADERS += sys/queue.h
 noinst_HEADERS += sys/socket.h
 noinst_HEADERS += sys/types.h

include/sys/cdefs.h

@@ -1,16 +0,0 @@
-/*
- * Public domain
- * sys/cdefs.h compatibility shim
- */
-
-#include_next <sys/cdefs.h>
-
-#ifndef LIBCOMPAT_SYS_CDEFS_H
-#define LIBCOMPAT_SYS_CDEFS_H
-
-#if !defined(HAVE_ATTRIBUTE__DEAD) && !defined(__dead)
-#define __dead          __attribute__((__noreturn__))
-#define __pure          __attribute__((__const__))
-#endif
-
-#endif

include/sys/types.h

@@ -14,6 +14,10 @@
 #include <_bsd_types.h>
 #endif
 
+#if !defined(HAVE_ATTRIBUTE__DEAD) && !defined(__dead)
+#define __dead		__attribute__((__noreturn__))
+#endif
+
 #if !defined(HAVE_ATTRIBUTE__BOUNDED__) && !defined(__bounded__)
 # define __bounded__(x, y, z)
 #endif