usbutils.pc.in: Fix Cflags entry

[fabioestevam]

When updating the usbutils version in OpenEmbedded from 015 to 017, the following QA error is seen:

QA Issue: File /usr/lib/pkgconfig/usbutils.pc in package usbutils-dev contains reference to TMPDIR [buildpaths]

As this causes reproducibility problem due to the host PC path being leaked, it is treated as error.

Fix it by using the standard Cflags entry.