libfdt: Upgrade libfdt source files

Update the libfdt source files, the upstream commit is 73e0f143b73d
("libfdt: fdt_strerror(): Fix comparison warning").

This brings us the fixes for the signed/unsigned comparison warnings,
so platforms can enable -Wsign-compare now.

Change-Id: I303d891c82ffea0acefdde27289339db5ac5a289
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
diff --git a/include/lib/libfdt/libfdt.h b/include/lib/libfdt/libfdt.h
index 48f375c..544d3ef 100644
--- a/include/lib/libfdt/libfdt.h
+++ b/include/lib/libfdt/libfdt.h
@@ -9,6 +9,10 @@
 #include <libfdt_env.h>
 #include <fdt.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #define FDT_FIRST_SUPPORTED_VERSION	0x02
 #define FDT_LAST_SUPPORTED_VERSION	0x11
 
@@ -2069,4 +2073,8 @@
 
 const char *fdt_strerror(int errval);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* LIBFDT_H */