externals: don't try install cubeb files

Install the project...
/usr/local/bin/cmake -P cmake_install.cmake
-- Install configuration: "Release"
-- Installing: /prefix/share/man/man6/citra.6
CMake Error at externals/cubeb/cmake_install.cmake:44 (file):
  file INSTALL cannot find "/path/to/citra/include".
Call Stack (most recent call first):
  externals/cmake_install.cmake:46 (include)
  cmake_install.cmake:49 (include)

gmake: *** [Makefile:74: install] Error 1

externals/CMakeLists.txt

@@ -81,5 +81,5 @@ endif()
 # Cubeb
 if(ENABLE_CUBEB)
     set(BUILD_TESTS OFF CACHE BOOL "")
-    add_subdirectory(cubeb)
+    add_subdirectory(cubeb EXCLUDE_FROM_ALL)
 endif()