cmake: Fix libnghttp2.pc paths

Must be absolute instead of relative

CMakeLists.txt

@@ -468,8 +468,8 @@ set(abs_top_builddir "${CMAKE_BINARY_DIR}")
 # libnghttp2.pc (pkg-config file)
 set(prefix          "${CMAKE_INSTALL_PREFIX}")
 set(exec_prefix     "${CMAKE_INSTALL_PREFIX}")
-set(libdir          "${CMAKE_INSTALL_LIBDIR}")
-set(includedir      "${CMAKE_INSTALL_INCLUDEDIR}")
+set(libdir          "${CMAKE_INSTALL_FULL_LIBDIR}")
+set(includedir      "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
 set(VERSION         "${PACKAGE_VERSION}")
 # For init scripts and systemd service file
 set(bindir          "${CMAKE_INSTALL_FULL_BINDIR}")