Add conditional build tag to exclude microphones for camera-only builds (#679)

* Add tag

* Add nomicrophone build tag for optional microphone support

- Changed from platform-specific (!windows) to opt-out build tag (!nomicrophone)
- Microphone support included by default (non-breaking)
- Use -tags nomicrophone to exclude when malgo deps unavailable
- Useful for cross-compilation and minimal builds
- Renamed microphone_windows.go to microphone_stub.go for clarity

* Update README

README.md

@@ -21,6 +21,16 @@
 go get -u github.com/pion/mediadevices
 ```
 
+### Build Tags
+
+`mediadevices` provides a `nomicrophone` build tag. Use this tag to build without microphone support. This is particularly useful for:
+- Cross-compilation where CGO dependencies like `malgo` are unavailable.
+- Projects where audio is not required.
+
+```bash
+go build -tags nomicrophone
+```
+
 ### Usage
 
 The following snippet shows how to capture a camera stream and store a frame as a jpeg image:

pkg/driver/microphone/microphone.go

@@ -1,3 +1,6 @@
+//go:build !nomicrophone
+// +build !nomicrophone
+
 package microphone
 
 import (

pkg/driver/microphone/microphone_stub.go

@@ -0,0 +1,15 @@
+//go:build nomicrophone
+// +build nomicrophone
+
+package microphone
+
+// This stub file is used when building with the 'nomicrophone' build tag.
+// Use this when cross-compiling or when malgo (miniaudio) dependencies are not available.
+//
+// To build without microphone support:
+//   go build -tags nomicrophone
+//
+// This is particularly useful for:
+// - Cross-compilation where CGO dependencies are unavailable
+// - Environments without audio system development libraries
+// - Minimal builds that only need camera/video support