patch 8.1.1616: build failure with gcc on Amiga

Problem:    Build failure with gcc on Amiga.
Solution:   Add missing header includes. (Ola Söder, closes #4603)

src/os_amiga.h

@@ -88,6 +88,10 @@ typedef long off_t;
 #ifdef __GNUC__
 # include <sys/stat.h>
 # include <unistd.h>
+# include <limits.h>
+# include <errno.h>
+# include <time.h>
+# include <dirent.h>
 #endif
 
 #ifndef PROTO