Fix a member variables initialization order issue

Summary:
In MacOS, I got issue with `Footer`'s default constructor, which initialized the magic number with some random number instead of 0.
With investigation, I found we forgot to make the kInvalidTableMagicNumber to be static. As a result, kInvalidTableMagicNumber was assgined to `table_magic_number_` before it is initialized (which will be populated with random number).

Test Plan: passed current unit tests; also passed the unit tests for the incoming diff which used the default footer.

Reviewers: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16077

Author: liukai

table/format.h

@@ -109,7 +109,7 @@ class Footer {
     kEncodedLength = 2 * BlockHandle::kMaxEncodedLength + 8
   };
 
-  const uint64_t kInvalidTableMagicNumber = 0;
+  static const uint64_t kInvalidTableMagicNumber = 0;
 
  private:
   // Set the table_magic_number only when it was not previously