Bug 1661274 - Unbreak Gtk build on non-Linux after bug 1652813;r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D88537

xpcom/base/nsMemoryReporterManager.h

@@ -182,10 +182,8 @@ class nsMemoryReporterManager final : public nsIMemoryReporterManager,
   static int64_t ResidentUnique(HANDLE aProcess = nullptr);
 #elif XP_MACOSX
   static int64_t ResidentUnique(mach_port_t aPort = 0);
-#elif XP_LINUX
-  static int64_t ResidentUnique(pid_t aPid = 0);
 #else
-  static int64_t ResidentUnique();
+  static int64_t ResidentUnique(pid_t aPid = 0);
 #endif  // XP_{WIN, MACOSX, LINUX, *}
 
   // Functions that measure per-tab memory consumption.