memory: move obsolete exec.c functions to a private header

This will help avoid accidental usage.

Signed-off-by: Avi Kivity <avi@redhat.com>
diff --git a/exec.c b/exec.c
index fbcb1bf..32782b4 100644
--- a/exec.c
+++ b/exec.c
@@ -57,6 +57,9 @@
 #include "trace.h"
 #endif
 
+#define WANT_EXEC_OBSOLETE
+#include "exec-obsolete.h"
+
 //#define DEBUG_TB_INVALIDATE
 //#define DEBUG_FLUSH
 //#define DEBUG_TLB