amigaos: add missing curl header [ci skip]

Without it, `CURLcode` and `CURLE_*` are undefined. `lib/hostip.h` and
conditional local code need them.

Reviewed-by: Daniel Stenberg

Closes #9761

lib/amigaos.c

@@ -26,6 +26,8 @@
 
 #ifdef __AMIGA__
 
+#include <curl/curl.h>
+
 #include "hostip.h"
 #include "amigaos.h"