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.

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
diff --git a/Makefile b/Makefile
index acbd00e..dd1c4f9 100644
--- a/Makefile
+++ b/Makefile
@@ -57,7 +57,7 @@
 libdir		= $(prefix)/$(LIBDIR)
 includedir	= $(prefix)/include
-pkgconfig_dir	= /usr/lib/pkgconfig
+pkgconfig_dir	= $(libdir)/pkgconfig