extmod: Add and reorganise compilation guards and includes.

To reduce dependencies on header files when extmod components are disabled.

Signed-off-by: Damien George <damien.p.george@gmail.com>

extmod/machine_timer.c

@@ -26,10 +26,11 @@
 
 #include "py/runtime.h"
 #include "py/mphal.h"
-#include "shared/runtime/softtimer.h"
 
 #if MICROPY_PY_MACHINE_TIMER
 
+#include "shared/runtime/softtimer.h"
+
 typedef soft_timer_entry_t machine_timer_obj_t;
 
 const mp_obj_type_t machine_timer_type;