Fix libdir in tesseract.pc from CMake

tesseract.pc.cmake was hardcoding libdir to
`{prefix}/lib`, which is wrong for systems that use
`/usr/lib64/` on 64-bit. `CMAKE_INSTALL_LIBDIR`
is already expected to contain the libdir path
relative to the install prefix.

tesseract.pc.cmake

@@ -1,6 +1,6 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}/bin
-libdir=${prefix}/lib
+libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
 includedir=${prefix}/include
 
 Name: @tesseract_NAME@