patch 8.0.1778: script to check translations does not always work

Problem:    Script to check translations does not always work.
Solution:   Go to first line before searching for MIME.
diff --git a/src/po/check.vim b/src/po/check.vim
index a1ec623..9cfa36a 100644
--- a/src/po/check.vim
+++ b/src/po/check.vim
@@ -121,6 +121,7 @@
 
 " Check that the \n at the end of the msgid line is also present in the msgstr
 " line.  Skip over the header.
+1
 /^"MIME-Version:
 while 1
   let lnum = search('^msgid\>')