build(install): Fix pkgconfig install directory

`pkg-config` finds configs within `$prefix/lib/pkgconfig`, but not in
`$prefix/lib`.

See-Also: https://linux.die.net/man/1/pkg-config

cmake/ftxui_install.cmake

@@ -58,5 +58,5 @@ install(
 configure_file(ftxui.pc.in ${CMAKE_CURRENT_BINARY_DIR}/ftxui.pc @ONLY)
 install(
   FILES ${CMAKE_CURRENT_BINARY_DIR}/ftxui.pc
-  DESTINATION ${CMAKE_INSTALL_LIBDIR}
+  DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig"
   )