libc: Update includes

Reflect the changes in the structure of libc.

NB: the include of stdarg.h in nvm_results_helpers.c is not in alphabetical
order because it needs to be included before stdio.h. Fixing this would require
further changes.

Change-Id: I07f62a3450802833408ff3e1f950fd3b643e5e33
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/include/common/debug.h b/include/common/debug.h
index 53eebf7..9b2d842 100644
--- a/include/common/debug.h
+++ b/include/common/debug.h
@@ -65,6 +65,12 @@
 # define VERBOSE(...)
 #endif
 
+#if ENABLE_BACKTRACE
+void backtrace(const char *cookie);
+#else
+#define backtrace(x)
+#endif
+
 /*
  * For the moment this panic function is very basic: report an error and
  * spin. This can be expanded in the future to provide more information.