Bug 1730747: Fix unused-var build warning in GMPProcessParent.cpp, by wrapping variable decl in the same ifdef as its usage. r=bryce

Differential Revision: https://phabricator.services.mozilla.com/D125589

dom/media/gmp/GMPProcessParent.cpp

@@ -32,7 +32,9 @@ using std::vector;
 using mozilla::gmp::GMPProcessParent;
 using mozilla::ipc::GeckoChildProcessHost;
 
+#ifdef MOZ_WIDGET_ANDROID
 static const int kInvalidFd = -1;
+#endif
 
 namespace mozilla::gmp {