This prevents a linking error since ntp_dispatch_imsg calls
constraint_reset().

Otherwise, systems that don't support it will treat it as a name for a
global `struct ntp_msg` variable.

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.