Fix GCC build error on Linux.

pcbnew/drc/drc_engine.cpp

@@ -23,6 +23,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <atomic>
 #include <thread>
 #include <reporter.h>
 #include <progress_reporter.h>

pcbnew/drc/drc_test_provider_disallow.cpp

@@ -21,6 +21,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <atomic>
 #include <thread>
 #include <common.h>
 #include <board_design_settings.h>

pcbnew/drc/drc_test_provider_sliver_checker.cpp

@@ -21,6 +21,7 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA
  */
 
+#include <atomic>
 #include <thread>
 #include <board.h>
 #include <board_design_settings.h>