gobject-introspection: Do not hardcode objdump name

Use OBJDUMP variable in the script, this helps in using the lddwrapper
with recipes which maybe using different objdump tools e.g.l
llvm-objdump or vice-versa

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>