libcap: Adjust the lfs file to place pkg-config files in the correct place

libcap: Adjust the lfs file to place pkg-config files in the correct place

- libcap places the files by default in /lib and not /usr/lib etc. To fix this libcap made
   a symlink for the library file from /lib to /usr/lib. However the .pc files were left
   in /lib/pkgconfig and not /usr/lib/pkgconfig and were therefore not found by the update
   of rng-tools which now required libcap to be found.
- Changed the prefix settings for libcap which placed the libraries and .pc files in the
   correct locations while keeping the executables in their existing location.
- This removed the need for symlinking /usr/lib/libcap.so to /lib/libcap.so.2.67 as the
   libraries are now placed in /usr/lib
- Installed the ipfire build with these changes into a vm system and confirmed that
   everything worked. Input from Michael Tremer that if ping worked then libcap was
   functioning correctly.
- The prefixes have to be applied to both make and make install to end up with the files
   in the correct places.

Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>