patch 8.1.2084: Amiga: cannot get the user name

Problem:    Amiga: cannot get the user name.
Solution:   Use getpwuid() if available. (Ola Söder, closes #4985)

src/os_amiga.h

@@ -90,6 +90,7 @@ typedef long off_t;
 # include <unistd.h>
 # include <limits.h>
 # include <errno.h>
+# include <pwd.h>
 # include <dirent.h>
 #endif