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)
diff --git a/src/os_amiga.h b/src/os_amiga.h
index 6f951fb..71841ac 100644
--- a/src/os_amiga.h
+++ b/src/os_amiga.h
@@ -90,6 +90,7 @@
 # include <unistd.h>
 # include <limits.h>
 # include <errno.h>
+# include <pwd.h>
 # include <dirent.h>
 #endif