extmod/modbtree: Move system includes within MICROPY_PY_BTREE guard.

Since commit d6d8722, modbtree.c is
included unconditionally in the build (if SRC_EXTMOD_C is used).  So guard
the includes of system headers files in case a target doesn't have them.

Signed-off-by: Damien George <damien@micropython.org>

extmod/modbtree.c

@@ -24,16 +24,14 @@
  * THE SOFTWARE.
  */
 
-#include <stdio.h>
-#include <string.h>
-#include <errno.h> // for declaration of global errno variable
-#include <fcntl.h>
-
 #include "py/runtime.h"
 #include "py/stream.h"
 
 #if MICROPY_PY_BTREE
 
+#include <stdio.h>
+#include <errno.h> // for declaration of global errno variable
+#include <fcntl.h>
 #include <db.h>
 #include <../../btree/btree.h>