C requires prototypes or compilers will complain about them missing. Our
library entry points do not have such, therefore add the macro
FREERDP_ENTRY_POINT which declares the function prototype automatically
before the function.
Building client channels out of tree does not work reliable as the
dependencies can not be easily split. (exceptions may be simple ones
as echo channel)
It does complicate the build system and code though, so remove this
for maintainability reasons.
