src: hide node::credentials::HasOnly outside unit

The function is not declared anywhere else and should not be visible
outside the compilation unit.

PR-URL: #50450
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>

src/node_credentials.cc

@@ -49,7 +49,7 @@ namespace credentials {
 
 #if defined(__linux__)
 // Returns true if the current process only has the passed-in capability.
-bool HasOnly(int capability) {
+static bool HasOnly(int capability) {
   DCHECK(cap_valid(capability));
 
   struct __user_cap_data_struct cap_data[2];