pnor_partition_table: Make writeUserdata() private to namespace

This is part of the ongoing effort to extract parseTocLine() from the
Table class, enabling reuse in setup for test cases.

Change-Id: I846e67b728df7bf57fd132b21438a13248d426fc
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/pnor_partition_table.cpp b/pnor_partition_table.cpp
index 997876d..9c9fb9c 100644
--- a/pnor_partition_table.cpp
+++ b/pnor_partition_table.cpp
@@ -96,7 +96,7 @@
     }
 }
 
-inline void Table::writeUserdata(pnor_partition& part, uint32_t version,
+static inline void writeUserdata(pnor_partition& part, uint32_t version,
                                  const std::string& data)
 {
     std::istringstream stream(data);