patch 8.2.4615: mapping with escaped bar does not work in :def function

Problem:    Mapping with escaped bar does not work in :def function. (Sergey
            Vlasov)
Solution:   Do not remove the backslash. (closes #10002)
diff --git a/src/syntax.c b/src/syntax.c
index 6683d2a..74687cb 100644
--- a/src/syntax.c
+++ b/src/syntax.c
@@ -4764,7 +4764,7 @@
      * filename to include.
      */
     eap->argt |= (EX_XFILE | EX_NOSPC);
-    separate_nextcmd(eap);
+    separate_nextcmd(eap, FALSE);
     if (*eap->arg == '<' || *eap->arg == '$' || mch_isFullName(eap->arg))
     {
 	// For an absolute path, "$VIM/..." or "<sfile>.." we ":source" the