patch 8.2.1640: Amiga: missing header for getgrgid()

Problem:    Amiga: missing header for getgrgid().
Solution:   Add the grp.h header. (Ola Söder, closes #6906)
diff --git a/src/os_amiga.h b/src/os_amiga.h
index 611555e..98f3cff 100644
--- a/src/os_amiga.h
+++ b/src/os_amiga.h
@@ -91,6 +91,7 @@
 # include <limits.h>
 # include <errno.h>
 # include <pwd.h>
+# include <grp.h>
 # include <dirent.h>
 #endif