src/Makefile.am: Add usb.h and usb.cpp files to powertop_SOURCES

Resolve this error:

In function `one_measurement(int, char*)':
undefined reference to `create_all_usb_devices()'
devices/powertop-device.o: In function `create_all_devices()':
undefined reference to `create_all_usb_devices()'

by adding devices/usb.h and devices/usb.cpp files to the
powertop_SOURCES list.

Signed-off-by: Lisa Nguyen <lisa.nguyen@linaro.org>
1 file changed