Don't install PDB for Windows static build of caffe2_observers (pytorch#16420)

Summary:
Fixes pytorch#16292.
Pull Request resolved: pytorch#16420

Differential Revision: D13833704

Pulled By: soumith

fbshipit-source-id: 482ad6ce103bed7206e924e8c82454fbb1bfac42

Author: peterjc123

modules/observers/CMakeLists.txt

@@ -19,7 +19,7 @@ target_include_directories(caffe2_observers PUBLIC ${CMAKE_CURRENT_SOURCE_DIR}/.
 target_compile_options(caffe2_observers PRIVATE "-DCAFFE2_BUILD_OBSERVER_LIB")
 install(TARGETS caffe2_observers DESTINATION lib)
 caffe2_interface_library(caffe2_observers caffe2_observers_library)
-if (MSVC)
+if (MSVC AND BUILD_SHARED_LIBS)
   install(FILES $<TARGET_PDB_FILE:caffe2_observers> DESTINATION lib OPTIONAL)
 endif()