[test] Add missing #include <string.h>

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/tests/rsa_test.c

 /* Forcibly enable assertions */
 #undef NDEBUG
 
+#include <string.h>
 #include <ipxe/crypto.h>
 #include <ipxe/rsa.h>
 #include <ipxe/md5.h>