Merge pull request #1 from ARMmbed/g-fix-size-const

Fix const attribute mismatch on size function

Author: simonqhughes

features/filesystem/sd/SDBlockDevice.h

@@ -149,7 +149,7 @@ class SDBlockDevice : public BlockDevice {
     unsigned _block_size;
     bool _is_initialized;
     bool _dbg;
-    Mutex _lock;
+    mutable Mutex _lock;
 };