Move include/malloc.h to include/gpxe/malloc.h, since everything in there

is now gPXE-specific.  (The standard malloc() et al have been in stdlib.h
for a while).

Add free memory counter.

src/core/heap.c

@@ -1,4 +1,4 @@
-#include <malloc.h>
+#include <gpxe/malloc.h>
 #include <gpxe/heap.h>
 
 /**