Fix build if libdwarf dir missing
Fix build if libdwarf dir missing

I was getting errors from -Werror=missing-include-dirs due to
$includedir/libdwarf not existing when builing in a different
prefix than /usr.

handsome-feng/flatpak,flatpak/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,matthiasclasen/flatpak,handsome-feng/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,flatpak/flatpak,matthiasclasen/flatpak