Guard Kaldi's version generation (#1715)

When building torchaudio from source, `get_version.sh` from kaldi is executed everytime,
which results in kaldi-bindings to be always rebuilt.

This commit add "if" guard to the part so that they are not always executed.

third_party/kaldi/CMakeLists.txt

 set(KALDI_REPO ${CMAKE_CURRENT_SOURCE_DIR}/submodule)
 
+if (NOT EXISTS ${KALDI_REPO}/src/base/version.h)
 # Apply custom patch
 execute_process(
   WORKING_DIRECTORY ${KALDI_REPO}
@@ -14,6 +15,7 @@ execute_process(
   WORKING_DIRECTORY ${KALDI_REPO}/src/base
   COMMAND sh get_version.sh
   )
+endif()
 
 set(KALDI_SOURCES
   src/matrix/kaldi-vector.cc