meson: Build fuzz executables by default if fuzz-tests option is enabled

fuzz-tests is enabled by default now but we still don't build the fuzz
executables by default. Let's change that so that we always make sure these
still compile when we make changes.

meson.build

@@ -2196,7 +2196,7 @@ test_additional_kwargs = {
 }
 
 fuzz_template = executable_template + {
-        'build_by_default' : fuzzer_build,
+        'build_by_default' : want_fuzz_tests,
         'install' : false,
 }