Pkgconfig install may result on incorrect path 

When installing rtl-entropy into /usr, the pkgconfig file rtl-entropy.pc gets incorrectly copied to /usr/pkgconfig. 

These small adjustments should make sure the installation path of rtl-entropy.pc match the path and routine used on rtl-sdr. (i.e. I've copied sections of the from rtl-sdr's CMakeLists.txt ... 8D ) 

fixes pwarren/rtl-entropy#6