Made get_header() static.

src/block_header.c

@@ -5,9 +5,9 @@
  *
  * \param ptr The data part of the memory block.
  */
-block_header_t * get_header(void *ptr);
+static block_header_t * get_header(void *ptr);
 
-block_header_t * get_header(void *ptr) {
+static block_header_t * get_header(void *ptr) {
     return (block_header_t *) ((char *) ptr - HEADER_SIZE);
 }