use libdir for .pc install

The .pc files should be under the same libdir as the libraries, so use
the existing $(libdir) variable rather than hardcoding /usr/lib/ when
installing the pkg-config files.

BUG=chromium-os:20636
TEST=`emerge-amd64-generic flashmap` and see .pc files in /usr/lib64/
TEST=`emerge-x86-generic flashmap` and see .pc files in /usr/lib/

Change-Id: I619f6d4c7d04eda0cbe0e3d0fed987a69d764021
1 file changed