Missing header files when compiling libmei (master branch and last release)

[martha-thomae]

Fails to build the mei framework, indicating that there are two missing files:

• libmei-master/src/modules/linkalignmixins.h
• libmei-master/src/modules/linkalign.h

This happens in master as well as in the last release (3.0.0)

[martha-thomae]

This seems to be the same issue as #100.