Add missing header files.

These headers files were missing from the header files that
I added them to; the fact that they were missing didn't
lead to compile errors because by coincidence the missing
headers are included in the source files before including
these headers. But, after the reordering of header inclusions
by Tools/fix_headers.sh, these cases will give rise to compiler
errors.

src/modules/systemlib/board_serial.h

@@ -42,6 +42,8 @@
 
 #pragma once
 
+#include <stdint.h>	// uint8_t
+
 __BEGIN_DECLS
 
 __EXPORT int get_board_serial(uint8_t *serialid);

src/modules/systemlib/cpuload.h

@@ -37,6 +37,7 @@
 
 #include <sched.h>
 #include <stdint.h>
+#include <stdbool.h>
 
 struct system_load_taskinfo_s {
 	uint64_t total_runtime;			///< Runtime since start (start_time - total_runtime)/(start_time - current_time) = load