AudioCommon: Use emplace_back instead of push_back in GetSoundBackends()

Constructs the strings directly within the container instead of
performing a construction, then a copy.

The reasoning is that the BACKEND_* strings are const char arrays, so
the push_back code is equivalent to:

push_back(std::string(BACKEND_WHATEVER)) instead of forwarding the
arguments to a constructed instance directly in the container.

Source/Core/AudioCommon/AudioCommon.cpp

@@ -99,20 +99,20 @@ std::vector<std::string> GetSoundBackends()
 {
   std::vector<std::string> backends;
 
-  backends.push_back(BACKEND_NULLSOUND);
-  backends.push_back(BACKEND_CUBEB);
+  backends.emplace_back(BACKEND_NULLSOUND);
+  backends.emplace_back(BACKEND_CUBEB);
   if (XAudio2_7::isValid() || XAudio2::isValid())
-    backends.push_back(BACKEND_XAUDIO2);
+    backends.emplace_back(BACKEND_XAUDIO2);
   if (AlsaSound::isValid())
-    backends.push_back(BACKEND_ALSA);
+    backends.emplace_back(BACKEND_ALSA);
   if (PulseAudio::isValid())
-    backends.push_back(BACKEND_PULSEAUDIO);
+    backends.emplace_back(BACKEND_PULSEAUDIO);
   if (OpenALStream::isValid())
-    backends.push_back(BACKEND_OPENAL);
+    backends.emplace_back(BACKEND_OPENAL);
   if (OpenSLESStream::isValid())
-    backends.push_back(BACKEND_OPENSLES);
+    backends.emplace_back(BACKEND_OPENSLES);
   if (WASAPIStream::isValid())
-    backends.push_back(BACKEND_WASAPI);
+    backends.emplace_back(BACKEND_WASAPI);
 
   return backends;
 }