libmusicbrainz: fix subsequent invocations of do_configure

Every second invocation of do_configure failed with this error:

| CMake Error at src/CMakeLists.txt:19 (INCLUDE):
|   include could not find load file:
|
|     [...]/git/build-native/ImportExecutables.cmake

Signed-off-by: Andreas Oberritter <obi@opendreambox.org>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>