build: Fix absl log flags being stripped

According to a comment in
packager/third_party/abseil-cpp/source/absl/log/CMakeLists.txt, many
linkers will strip the contents of absl::log_flags because its symbols
symbols are only used in a global constructor, and that for now,
clients should link using
$<LINK_LIBRARY:WHOLE_ARCHIVE,absl::log_flags>.

Closes shaka-project#1325

packager/CMakeLists.txt

@@ -177,7 +177,8 @@ target_link_libraries(packager
   absl::flags
   absl::flags_parse
   absl::log
-  absl::log_flags
+  # See https://github.com/abseil/abseil-cpp/blob/c14dfbf9/absl/log/CMakeLists.txt#L464-L467
+  $<LINK_LIBRARY:WHOLE_ARCHIVE,absl::log_flags>
   absl::strings
   hex_bytes_flags
   libpackager
@@ -193,7 +194,8 @@ target_link_libraries(mpd_generator
   absl::flags
   absl::flags_parse
   absl::log
-  absl::log_flags
+  # See https://github.com/abseil/abseil-cpp/blob/c14dfbf9/absl/log/CMakeLists.txt#L464-L467
+  $<LINK_LIBRARY:WHOLE_ARCHIVE,absl::log_flags>
   absl::strings
   license_notice
   mpd_builder