tst-rec-dlopen: Fix build fail due to missing inclusion of string.h

on S390, I get a compile error for dlfcn/tst-rec-dlopen.c:
tst-rec-dlopen.c: In function ‘malloc’:
tst-rec-dlopen.c:101:4: error: implicit declaration of function ‘strlen’ [-Werror=implicit-function-declaration]
    (void) write (STDOUT_FILENO, message, strlen (message));
    ^
tst-rec-dlopen.c:101:42: error: incompatible implicit declaration of built-in function ‘strlen’ [-Werror]
    (void) write (STDOUT_FILENO, message, strlen (message));
                                          ^
tst-rec-dlopen.c:112:42: error: incompatible implicit declaration of built-in function ‘strlen’ [-Werror]
    (void) write (STDOUT_FILENO, message, strlen (message));
                                          ^

This patch adds the missing "#include <string.h>" for strlen.

ChangeLog:

	* dlfcn/tst-rec-dlopen.c: Include string.h.