[meson] Enabled needlessly disabled MSVC warnings

We don’t seem to hot any of these warnings currently.

meson.build

@@ -28,10 +28,7 @@ if cpp.get_argument_syntax() == 'msvc'
   # If a warning is harmless but hard to fix, use '/woXXXX' so it's shown once
   # NOTE: Only add warnings here if you are sure they're spurious
   msvc_args = [
-    '/wd4018', # implicit signed/unsigned conversion
-    '/wd4146', # unary minus on unsigned (beware INT_MIN)
     '/wd4244', # lossy type conversion (e.g. double -> int)
-    '/wd4305', # truncating type conversion (e.g. double -> float)
     cpp.get_supported_arguments(['/utf-8']), # set the input encoding to utf-8
   ]
   add_project_arguments(msvc_args, language: ['c', 'cpp'])