Merge individual Battery.[ch] files into Platform.[ch]

Consistent with everything else involving platform-specific
calls from core htop code.

linux/Platform.h

@@ -11,6 +11,7 @@ in the source distribution for its full text.
 #include <sys/types.h>
 
 #include "Action.h"
+#include "BatteryMeter.h"
 #include "DiskIOMeter.h"
 #include "Meter.h"
 #include "Process.h"
@@ -62,4 +63,6 @@ bool Platform_getNetworkIO(unsigned long int* bytesReceived,
                            unsigned long int* bytesTransmitted,
                            unsigned long int* packetsTransmitted);
 
+void Platform_getBattery(double *percent, ACPresence *isOnAC);
+
 #endif