AP_HAL_Linux: fix leaking strings

libraries/AP_HAL_Linux/PWM_Sysfs.cpp

@@ -46,8 +46,10 @@ PWM_Sysfs_Base::~PWM_Sysfs_Base()
 {
     ::close(_duty_cycle_fd);
 
+    free(_export_path);
     free(_polarity_path);
     free(_enable_path);
+    free(_duty_path);
     free(_period_path);
 }