Fixed const correctness issues that have no impact on the ABI
(cherry picked from commit eae09db9e57b7a342ea15bf57c5c1439c59a2e50)

Conflicts:
	library/gcm.c
diff --git a/include/polarssl/error.h b/include/polarssl/error.h
index a084243..728b29e 100644
--- a/include/polarssl/error.h
+++ b/include/polarssl/error.h
@@ -27,6 +27,8 @@
 #ifndef POLARSSL_ERROR_H
 #define POLARSSL_ERROR_H
 
+#include <string.h>
+
 /**
  * Error code layout.
  *