Fix Windows build(external/coinmp)

This patch fixes the problem that a linker misses CoinMP.dll.
The problem will occurs when we use --enable-dbgutil option.

Change-Id: If6cec0f9705db2af33cb9a69d9440e3053c5979a
Reviewed-on: https://gerrit.libreoffice.org/8854
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>