firmware: fix missing includes in some firmware headers

Add includes for any types/macros we use in these headers.

Change-Id: Icfcd7a58376c5738fe6562cdb3cdd7855df35c75

firmware/export/backlight.h

@@ -22,7 +22,7 @@
 #define BACKLIGHT_H
 
 #include "config.h"
-
+#include <stdbool.h>
 
 
 #if !defined(BOOTLOADER)

firmware/export/pcm-internal.h

@@ -23,6 +23,7 @@
 #define PCM_INTERNAL_H
 
 #include "config.h"
+#include "gcc_extensions.h" /* for FORCE_INLINE */
 
 #ifdef HAVE_SW_VOLUME_CONTROL
 /* Default settings - architecture may have other optimal values */

firmware/export/pcm.h

@@ -23,6 +23,7 @@
 
 #include <string.h> /* size_t */
 #include <inttypes.h> /* uint32_t */
+#include <stdbool.h>
 #include "config.h"
 
 enum pcm_dma_status

firmware/export/power.h

@@ -22,6 +22,7 @@
 #define _POWER_H_
 
 #include "config.h"
+#include <stdbool.h>
 
 #if CONFIG_CHARGING
 enum power_input_flags {