Move include(CTest) to inside testing guard

The RTAUDIO_BUILD_TESTING variable stops the testing target ejection
but still does an include(CTest) which injects the standard CTest
macros from enable_testing(). In rtmidi you include(CTest) inside
the guard but in rtaudio you do not; as such using the cmake in
a non-ctest project ejects multiple targets.

For now you can get around this with BUILD_TESTING globally off
but that means you can't activate your own ctests optionally.

So move the include(CTEst) to be in the same spot as it is in
rtmidi.

Thanks for great software!

CMakeLists.txt

@@ -280,12 +280,12 @@ target_compile_definitions(rtaudio PRIVATE RTAUDIO_EXPORT)
 target_link_libraries(rtaudio ${LINKLIBS})
 
 # Subdirs
-include(CTest)
 
 if (NOT DEFINED RTAUDIO_BUILD_TESTING OR RTAUDIO_BUILD_TESTING STREQUAL "")
   set(RTAUDIO_BUILD_TESTING ${BUILD_TESTING})
 endif()
 if (RTAUDIO_BUILD_TESTING)
+  include(CTest)
   add_subdirectory(tests)
 endif()