rtld.1: Improve _rtld_functrace example

The man page was written at a time that gcc was "leaking" headers.
Now <unistd.h> is required for this example to compile.

libexec/rtld-elf/rtld.1

@@ -302,6 +302,7 @@ To set up an
 for printing out the functions as they are called, this code can be used:
 .Bd -literal -offset indent
 #include <string.h>
+#include <unistd.h>
 
 static int nl = 10;