tools/power turbostat: Fix bogus SysWatt for forked program

Similar to delta_cpu(), delta_platform() is called in turbostat main
loop. This ensures accurate SysWatt readings in periodic monitoring mode
$ sudo turbostat -S -q --show power -i 1
CoreTmp	PkgTmp	PkgWatt	CorWatt	GFXWatt	RAMWatt	PKG_%	RAM_%	SysWatt
60	61	6.21	1.13	0.16	0.00	0.00	0.00	13.07
58	61	6.00	1.07	0.18	0.00	0.00	0.00	12.75
58	61	5.74	1.05	0.17	0.00	0.00	0.00	12.22
58	60	6.27	1.11	0.24	0.00	0.00	0.00	13.55

However, delta_platform() is missing for forked program and causes bogus
SysWatt reporting,
$ sudo turbostat -S -q --show power sleep 1
1.004736 sec
CoreTmp	PkgTmp	PkgWatt	CorWatt	GFXWatt	RAMWatt	PKG_%	RAM_%	SysWatt
57	58	6.05	1.02	0.16	0.00	0.00	0.00	0.03

Add missing delta_platform() for forked program.

Fixes: e5f687b8 ("tools/power turbostat: Add RAPL psys as a built-in counter")
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>