MDEV-19845: Fix the build on some platforms

On some platforms, MY_RELAX_CPU() falls back to an atomic
memory operation, but my_cpu.h fails to include my_atomic.h.

mysys/my_cpu.c

@@ -14,6 +14,7 @@
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301  USA */
 
 #include <my_global.h>
+#include <my_atomic.h>
 #include <my_cpu.h>
 
 #ifdef HAVE_PAUSE_INSTRUCTION

storage/innobase/include/ib0mutex.h

@@ -29,6 +29,7 @@ Created 2013-03-26 Sunny Bains.
 #ifndef ib0mutex_h
 #define ib0mutex_h
 
+#include "my_atomic.h"
 #include "my_cpu.h"
 #include "os0event.h"
 #include "sync0arr.h"