libc: Import strlcat from FreeBSD project

From commit: 21571b1d140ae7bb44e94c0afba2ec61456b275b
Made small changes to fit into TF-A project

Change-Id: I07fd7fe1037857f6b299c35367c104fb51fa5cfa
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>
diff --git a/include/lib/libc/string.h b/include/lib/libc/string.h
index 71774b0..91cbafb 100644
--- a/include/lib/libc/string.h
+++ b/include/lib/libc/string.h
@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: BSD-3-Clause
  */
 /*
- * Portions copyright (c) 2018-2019, ARM Limited and Contributors.
+ * Portions copyright (c) 2018-2020, ARM Limited and Contributors.
  * All rights reserved.
  */
 
@@ -26,5 +26,6 @@
 size_t strnlen(const char *s, size_t maxlen);
 char *strrchr(const char *p, int ch);
 size_t strlcpy(char * dst, const char * src, size_t dsize);
+size_t strlcat(char * dst, const char * src, size_t dsize);
 
 #endif /* STRING_H */