src: add a missing space in node_os.cc

This commit makes a small stylistic fix adding a missing space in
GetOSType function in node_os.cc

PR-URL: https://github.com/nodejs/node/pull/10931
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michal Zasso <targos@protonmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

src/node_os.cc

@@ -69,7 +69,7 @@ static void GetOSType(const FunctionCallbackInfo<Value>& args) {
   }
   rval = info.sysname;
 #else  // __MINGW32__
-  rval ="Windows_NT";
+  rval = "Windows_NT";
 #endif  // __POSIX__
 
   args.GetReturnValue().Set(OneByteString(env->isolate(), rval));