[healthkit] Add missing static to GetClinicalType (#4657)

*Note: Credit to Paul DiPietro (@pauldipietro) for finding this when trying to use the new HealthKit APIs.*

src/healthkit.cs

@@ -954,6 +954,7 @@ namespace HealthKit {
 		HKClinicalType GetClinicalType (NSString identifier);
 
 		[Watch (5,0), iOS (12,0)]
+		[Static]
 		[Wrap ("GetClinicalType (identifier.GetConstant ())")]
 		HKClinicalType GetClinicalType (HKClinicalTypeIdentifier identifier);
 	}