usr.bin/dtc: Include <limits> for std::numeric_limits<>.

This is needed when building natively as a cross-tool on hosts such as
Linux using more recent versions of libstdc++.

Co-authored-by:	Alexander Richardson <arichardson@FreeBSD.org>
Obtained from:	CheriBSD

usr.bin/dtc/fdt.cc

@@ -38,6 +38,7 @@
 #include "dtb.hh"
 
 #include <algorithm>
+#include <limits>
 #include <sstream>
 
 #include <ctype.h>

usr.bin/dtc/input_buffer.cc

@@ -35,7 +35,6 @@
 #include "input_buffer.hh"
 #include <ctype.h>
 #include <errno.h>
-#include <limits.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
@@ -44,6 +43,7 @@
 #ifndef NDEBUG
 #include <iostream>
 #endif
+#include <limits>
 
 
 #include <sys/stat.h>