Add missing header

Fix build issue:

```
/tmp/libfreefare/libfreefare/felica.c:50:16: error: implicitly declaring library
      function 'malloc' with type 'void *(unsigned long)'
      [-Werror,-Wimplicit-function-declaration]
    if ((tag = malloc(sizeof(struct felica_tag)))) {
               ^
/tmp/libfreefare/libfreefare/felica.c:50:16: note: include the header <stdlib.h>
      or explicitly provide a declaration for 'malloc'
/tmp/libfreefare/libfreefare/felica.c:64:5: error: implicit declaration of
      function 'free' is invalid in C99
      [-Werror,-Wimplicit-function-declaration]
    free(tag);
    ^
2 errors generated.
```

libfreefare/felica.c

@@ -13,6 +13,7 @@
 
 #include <assert.h>
 #include <errno.h>
+#include <stdlib.h>
 #include <string.h>
 
 #include <sys/types.h>