Platform/Haiku: Remove the include-once behavior

The behavior was added by commit 7ebc1cb2 (Haiku: Several fixes to
platform module, 2013-10-05, v3.0.0-rc1~541^2~3), but was not explained
in its commit message.  No other platform modules do this.

The include-once behavior results in incorrect variables set in some
larger projects, e.g. .NET Core, particularly when cross-compiling,
`CMAKE_DL_LIBS` being one of these.  Remove it.

Modules/Platform/Haiku.cmake

-# process only once
-if(HAIKU)
-  return()
-endif()
-
 set(HAIKU 1)
 set(UNIX 1)