meson: Downgrade unused function from error to warning in local builds

When working on systemd, it's often useful to be able to comment out
a function to see how a build behaves without it. Currently, when doing
this with a static function that's only used once, the build fails because
the function then becomes unused. As such, Let's downgrade the unused
function error to a warning in local builds.

meson.build

@@ -348,7 +348,6 @@ possible_common_cc_flags = [
         '-Werror=shift-count-overflow',
         '-Werror=shift-overflow=2',
         '-Werror=undef',
-        '-Werror=unused-function',
         '-Wfloat-equal',
         '-Wimplicit-fallthrough=5',
         '-Winit-self',
@@ -363,6 +362,7 @@ possible_common_cc_flags = [
         '-Wstrict-aliasing=2',
         '-Wstrict-prototypes',
         '-Wsuggest-attribute=noreturn',
+        '-Wunused-function',
         '-Wwrite-strings',
 
         # negative arguments are correctly detected starting with meson 0.46.