Bluetooth: shell: Make cmd_gatt_mread static

This function is not used outside of main.c.

Change-Id: Ia8cc6c2b2193906dd77d031b73d289c6acd128b2
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>

tests/bluetooth/shell/src/main.c

@@ -788,7 +788,7 @@ static void cmd_gatt_read(int argc, char *argv[])
 	}
 }
 
-void cmd_gatt_mread(int argc, char *argv[])
+static void cmd_gatt_mread(int argc, char *argv[])
 {
 	uint16_t h[8];
 	int i, err;