fix cmake error: no pdb file when BUILD_STATIC on windows (#780)

Author: xflcx1991

CMakeLists.txt

@@ -274,7 +274,9 @@ if(BUILD_STATIC)
 endif()
 
 if(WIN32 AND NOT MINGW)
-    install(FILES $<TARGET_PDB_FILE:${PROJECT_NAME}> DESTINATION bin OPTIONAL)
+    if(BUILD_SHARED)
+        install(FILES $<TARGET_PDB_FILE:${PROJECT_NAME}> DESTINATION bin OPTIONAL)
+    endif()
 endif()
 
 install(FILES ${LIBHV_HEADERS} DESTINATION include/hv)