powertop: needs autoreconf

powertop: needs autoreconf

Commit 01048deab60 (package/powertop: bump to version 2.9) removed
autoreconf even though the remaining patch touches Makefile.am. Fix
this.

Fixes:
http://autobuild.buildroot.net/results/3bf/3bf8daee559e2978f8602c56397afda771299032/
http://autobuild.buildroot.net/results/f9b/f9b4f113a333107a972fa51bd0347eeb0d52496f/
http://autobuild.buildroot.net/results/067/0675826888b6f89e76291c7835839344300ca9f5/

Cc: Romain Naour <romain.naour@gmail.com>
Cc: Steven Noonan <steven@uplinklabs.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>