examples: Fixed compilation warnings

pop3-multi.c:96:5: warning: implicit declaration of function 'memset'
imap-multi.c:96:5: warning: implicit declaration of function 'memset'
http2-download.c:226:5: warning: implicit declaration of function 'memset'
http2-upload.c:290:5: warning: implicit declaration of function 'memset'
http2-upload.c:290:5: warning: implicit declaration of function 'memset'

docs/examples/http2-download.c

@@ -25,6 +25,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 /* somewhat unix-specific */
 #include <sys/time.h>

docs/examples/http2-serverpush.c

@@ -25,6 +25,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 
 /* somewhat unix-specific */
 #include <sys/time.h>

docs/examples/http2-upload.c

@@ -25,6 +25,7 @@
  */
 #include <stdio.h>
 #include <stdlib.h>
+#include <string.h>
 #include <fcntl.h>
 #include <sys/stat.h>
 

docs/examples/imap-multi.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -20,6 +20,7 @@
  *
  ***************************************************************************/
 #include <stdio.h>
+#include <string.h>
 #include <curl/curl.h>
 
 /* This is a simple example showing how to fetch mail using libcurl's IMAP

docs/examples/pop3-multi.c

@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2015, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -20,6 +20,7 @@
  *
  ***************************************************************************/
 #include <stdio.h>
+#include <string.h>
 #include <curl/curl.h>
 
 /* This is a simple example showing how to retrieve mail using libcurl's POP3