APPS: Restore inclusions

An '#include <string.h>' was mistakenly removed from apps/ec.c and
apps/ecparam.c

Fixes #13986

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13989)

apps/ec.c

@@ -7,6 +7,7 @@
  * https://www.openssl.org/source/license.html
  */
 
+#include <string.h>
 #include <openssl/opensslconf.h>
 #include <openssl/evp.h>
 #include <openssl/encoder.h>

apps/ecparam.c

@@ -8,6 +8,7 @@
  * https://www.openssl.org/source/license.html
  */
 
+#include <string.h>
 #include <openssl/opensslconf.h>
 #include <openssl/evp.h>
 #include <openssl/encoder.h>