AP_HAL: mark semaphore take operations as WARN_IF_UNUSED

this prevents common bugs

libraries/AP_HAL/Semaphores.h

@@ -8,8 +8,8 @@
 
 class AP_HAL::Semaphore {
 public:
-    virtual bool take(uint32_t timeout_ms) = 0;
-    virtual bool take_nonblocking() = 0;
+    virtual bool take(uint32_t timeout_ms) WARN_IF_UNUSED = 0 ;
+    virtual bool take_nonblocking() WARN_IF_UNUSED = 0;
     virtual bool give() = 0;
 };