Fix warning of nested extern declaration of fn get_sys_time_float (#3499)

sw/airborne/arch/stm32/modules/core/threads_arch.c

@@ -13,7 +13,7 @@
 #include "modules/core/threads.h"
 #include "modules/core/threads_arch.h"
 #include "stdbool.h"
-
+#include "mcu_periph/sys_time.h"
 
 int pprz_mtx_init(pprz_mutex_t* mtx) {
   (void)mtx;
@@ -36,8 +36,6 @@ int pprz_mtx_unlock(pprz_mutex_t* mtx) {
   return 0;
 }
 
-
-
 void pprz_bsem_init(pprz_bsem_t* bsem, bool taken) {
   bsem->value = taken ? 0: 1;
 }