[PATCH]: tree-walk.h: Warning fix

Code that is using libgit.a, and hence including GIT's headers, may
get this warning:

"""
../tree-walk.h: In function 'tree_entry_len':
../tree-walk.h:25: warning: cast discards qualifiers from pointer target type
../tree-walk.h:25: warning: cast discards qualifiers from pointer target type
"""

This happens because the cast used in tree_entry_len() is discarding
the const qualifier (thanks to Shawn Pearce by noticing this).

Signed-off-by: Luiz Fernando N. Capitulino <redacted>

diff --git a/tree-walk.h b/tree-walk.h
index ee747ab..625198f 100644
--- a/tree-walk.h
+++ b/tree-walk.h

@@ -22,7 +22,7 @@ static inline const unsigned char *tree_entry_extract(struct tree_desc *desc, co
 
 static inline int tree_entry_len(const char *name, const unsigned char *sha1)
 {
-	return (char *)sha1 - (char *)name - 1;
+	return (const char *)sha1 - (const char *)name - 1;
 }
 
 void update_tree_entry(struct tree_desc *);