Fix non-standard import (#4470)

Author: paulb777

Firebase/Core/FIRHeartbeatInfo.m

@@ -12,7 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-#import "FIRHeartbeatInfo.h"
+#import "Private/FIRHeartbeatInfo.h"
 #import <GoogleUtilities/GULHeartbeatDateStorage.h>
 #import <GoogleUtilities/GULLogger.h>