patch 8.1.0741: viminfo with Blob is not tested

Problem:    Viminfo with Blob is not tested.
Solution:   Extend the viminfo test.  Fix reading a blob.  Fixed storing a
            special variable value.
diff --git a/src/proto/blob.pro b/src/proto/blob.pro
index 5e457dd..1c64524 100644
--- a/src/proto/blob.pro
+++ b/src/proto/blob.pro
@@ -10,4 +10,6 @@
 int blob_equal(blob_T *b1, blob_T *b2);
 int read_blob(FILE *fd, blob_T *blob);
 int write_blob(FILE *fd, blob_T *blob);
+char_u *blob2string(blob_T *blob, char_u **tofree, char_u *numbuf);
+blob_T *string2blob(char_u *str);
 /* vim: set ft=c : */