ircd: Include unistd.h in stdinc stack; remove the syscall() fwddecl.

include/ircd/stdinc.h

@@ -34,6 +34,7 @@ extern "C"
 {
 	#include <RB_INC_ASSERT_H
 	#include <RB_INC_STDARG_H
+	#include <RB_INC_UNISTD_H
 	#include <RB_INC_SYS_TIME_H
 	#include <RB_INC_SYS_UTSNAME_H
 }

include/ircd/util/syscall.h

@@ -11,9 +11,6 @@
 #pragma once
 #define HAVE_IRCD_UTIL_SYSCALL_H
 
-// Declaring this here eliminates the need to include <unistd.h> for it
-extern "C" long syscall(long, ...) noexcept;
-
 namespace ircd::util
 {
 	template<class function, class... args> long syscall(function&& f, args&&... a);