meson: allow fuzzers to be built even if fuzz testing is disabled

This makes commands like 'ninja -C build fuzz-journal-remote' or
'ninja -C build fuzzers' work, even if we have -Dfuzz-tests=false.
Two advantages: correctness of the meson declarations is verified even
if fuzzers are not built, and it easier to do a one-off build to check for
regressions or such.

Follow-up for 1763ef1d49.

meson.build

@@ -3438,7 +3438,6 @@ endif
 
 fuzzer_exes = []
 
-if fuzz_tests or fuzzer_build
 foreach tuple : fuzzers
         sources = tuple[0]
         link_with = tuple[1].length() > 0 ? tuple[1] : [libshared]
@@ -3469,9 +3468,9 @@ if fuzz_tests or fuzzer_build
                 dependencies : dependencies,
                 c_args : defs,
                 link_args: link_args,
-                        install : false)
+                install : false,
+                build_by_default : fuzz_tests or fuzzer_build)
 endforeach
-endif
 
 run_target(
         'fuzzers',