Auto merge of #477 - n-eq:processinfo-uptime, r=jrmuizel

Add ProcessInfo.systemUptime implementation

Hello,

Added [NSProcessInfo.systemUptime](https://developer.apple.com/documentation/foundation/processinfo/1414553-systemuptime) implementation, successfully tested on iOS.
Feel free to merge.

Best,

Author: bors-servo

cocoa-foundation/src/foundation.rs

@@ -229,6 +229,7 @@ pub trait NSProcessInfo: Sized {
         msg_send![class!(NSProcessInfo), processInfo]
     }
 
+    unsafe fn systemUptime(self) -> NSTimeInterval;
     unsafe fn processName(self) -> id;
     unsafe fn operatingSystemVersion(self) -> NSOperatingSystemVersion;
     unsafe fn isOperatingSystemAtLeastVersion(self, version: NSOperatingSystemVersion) -> bool;
@@ -239,6 +240,10 @@ impl NSProcessInfo for id {
         msg_send![self, processName]
     }
 
+    unsafe fn systemUptime(self) -> NSTimeInterval {
+        msg_send![self, systemUptime]
+    }
+
     unsafe fn operatingSystemVersion(self) -> NSOperatingSystemVersion {
         msg_send![self, operatingSystemVersion]
     }