os,ci: use a conditional include, instead of a platform file, to include libproc.h on macos only

vlib/os/os.c.v

@@ -5,6 +5,10 @@ import strings
 #include <sys/stat.h> // #include <signal.h>
 #include <errno.h>
 
+$if macos {
+	#include <libproc.h>
+}
+
 $if freebsd || openbsd {
 	#include <sys/sysctl.h>
 }

vlib/os/os_darwin.c.v

@@ -3,8 +3,6 @@
 // that can be found in the LICENSE file.
 module os
 
-#include <libproc.h>
-
 pub const sys_write = 4
 pub const sys_open = 5
 pub const sys_close = 6