Link pthread when building USE_PULSE.

This was working accidentally by picking up pthread via the common
configuration including USE_ALSA.

CMakeLists.txt

@@ -156,7 +156,7 @@ if(USE_PULSE)
   target_sources(cubeb PRIVATE
     src/cubeb_pulse.c)
   target_compile_definitions(cubeb PRIVATE USE_PULSE)
-  target_link_libraries(cubeb PRIVATE pulse)
+  target_link_libraries(cubeb PRIVATE pulse pthread)
   if(${CMAKE_SYSTEM_NAME} STREQUAL "Linux")
     target_link_libraries(cubeb PRIVATE dl)
   endif()