linux build: Disable a warning that url_launcher_linux can trigger

This works around the following issue in url_launcher_linux:
  flutter/flutter#153083

The issue only appears on some hosts, namely those with
"older versions of glib".

Author: gnprice

linux/CMakeLists.txt

@@ -41,7 +41,7 @@ endif()
 # of modifying this function.
 function(APPLY_STANDARD_SETTINGS TARGET)
   target_compile_features(${TARGET} PUBLIC cxx_std_14)
-  target_compile_options(${TARGET} PRIVATE -Wall -Werror)
+  target_compile_options(${TARGET} PRIVATE -Wall -Werror -Wno-unused-function)
   target_compile_options(${TARGET} PRIVATE "$<$<NOT:$<CONFIG:Debug>>:-O3>")
   target_compile_definitions(${TARGET} PRIVATE "$<$<NOT:$<CONFIG:Debug>>:NDEBUG>")
 endfunction()