Include <stdlib.h> in argp examples.

manual/examples/argp-ex1.c

@@ -5,6 +5,7 @@
    messages and exit when there are any arguments, and print
    a (rather pointless) messages for --help.  */
 
+#include <stdlib.h>
 #include <argp.h>
 
 int main (int argc, char **argv)

manual/examples/argp-ex2.c

@@ -19,6 +19,7 @@
    for a given program, even if it uses different argument
    parsers for various tasks).  */
 
+#include <stdlib.h>
 #include <argp.h>
 
 const char *argp_program_version =

manual/examples/argp-ex3.c

@@ -52,6 +52,7 @@
 
    An options vector should be terminated by an option with all fields zero. */
 
+#include <stdlib.h>
 #include <argp.h>
 
 const char *argp_program_version =