patch 8.1.0914: code related to findfile() is spread out

Problem:    Code related to findfile() is spread out.
Solution:   Put findfile() related code into a new source file. (Yegappan
            Lakshmanan, closes #3934)
diff --git a/src/proto.h b/src/proto.h
index d7e6316..1ab5cf7 100644
--- a/src/proto.h
+++ b/src/proto.h
@@ -80,6 +80,7 @@
 # include "ex_eval.pro"
 # include "ex_getln.pro"
 # include "fileio.pro"
+# include "findfile.pro"
 # include "fold.pro"
 # include "getchar.pro"
 # ifdef FEAT_HANGULIN