aom_timer.h: Don't include aom/aom_integer.h

aom_ports/aom_timer.h includes aom/aom_integer.h for NULL and int64_t.
These are defined in <stddef.h> and <stdint.h>, respectively, so include
those two headers instead. Also only need to include those two headers
in the #if CONFIG_OS_SUPPORT block.

Change-Id: I31bf13c43d7ea54e49bbc32ad3697d9b8d42c9b9
diff --git a/aom_ports/aom_timer.h b/aom_ports/aom_timer.h
index 9b17b89..9a876eb 100644
--- a/aom_ports/aom_timer.h
+++ b/aom_ports/aom_timer.h
@@ -14,10 +14,11 @@
 
 #include "config/aom_config.h"
 
-#include "aom/aom_integer.h"
-
 #if CONFIG_OS_SUPPORT
 
+#include <stddef.h>
+#include <stdint.h>
+
 #if defined(_WIN32)
 /*
  * Win32 specific includes