Make primary clipboard Linux only (#11843)

I guess only Linux supports the primary clipboard.

Release Notes:

- N/A

crates/gpui/src/app.rs

@@ -547,6 +547,7 @@ impl AppContext {
 
     /// Writes data to the primary selection buffer.
     /// Only available on Linux.
+    #[cfg(target_os = "linux")]
     pub fn write_to_primary(&self, item: ClipboardItem) {
         self.platform.write_to_primary(item)
     }
@@ -558,6 +559,7 @@ impl AppContext {
 
     /// Reads data from the primary selection buffer.
     /// Only available on Linux.
+    #[cfg(target_os = "linux")]
     pub fn read_from_primary(&self) -> Option<ClipboardItem> {
         self.platform.read_from_primary()
     }