patch 8.1.1279: cannot set 'spellang' to "sr@latin"

Problem:    Cannot set 'spellang' to "sr@latin". (Bojan Stipic)
Solution:   Allow using '@' in 'spellang'. (closes #4342)
diff --git a/src/option.c b/src/option.c
index 9583aa3..657c011 100644
--- a/src/option.c
+++ b/src/option.c
@@ -6057,7 +6057,7 @@
     int
 valid_spellang(char_u *val)
 {
-    return valid_name(val, ".-_,");
+    return valid_name(val, ".-_,@");
 }
 
 /*