Mark env-less connection objects as deprecated

This patch is the counterpart of the commit "Warn about the use of
deprecated env() function" in Genode repository:

  a7f40b24ca

include/gpio_session/zynq/connection.h

@@ -21,7 +21,7 @@ namespace Gpio {
 
 	struct Connection : Genode::Connection<Session>, Session_client
 	{
-        Connection(unsigned gpio_number)
+        Connection(unsigned gpio_number) __attribute__((deprecated))
         : Genode::Connection<Session>(session("ram_quota=8K, gpio=%zd", gpio_number)),
 		  Session_client(cap()) { }
 	};

include/i2c_session/connection.h

@@ -25,7 +25,8 @@ namespace I2C {
 	                   public Session_client
 	{
 	public:
-		Connection(unsigned int bus_num) :
+		Connection(unsigned int bus_num) __attribute__((deprecated))
+		:
 			Genode::Connection<Session>(session("ram_quota=4K, bus=%zd", bus_num)),
 			Session_client(cap()) { }
 	};

include/vdma_session/connection.h

@@ -21,7 +21,7 @@ namespace Vdma {
 
 	struct Connection : Genode::Connection<Session>, Session_client
 	{
-        Connection(unsigned vdma_number)
+        Connection(unsigned vdma_number) __attribute__((deprecated))
         : Genode::Connection<Session>(session("ram_quota=8K, vdma=%zd", vdma_number)),
 		  Session_client(cap()) { }
 	};