Remove dllexport/dllimport declspec from libmodplug headers

libmodplug now require stripping of the decoration for DLL
imports/exports, the easiest way to achieve it is a pair of sed commands
in the script.

Fix analogous to a commit to a similiar script that solves the problem:
https://github.com/jb-alvarado/media-autobuild_suite/commit/c6ca335008