env: Move env_get() to env.h

Move env_get() over to the new header file.

Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/hisilicon/poplar/poplar.c b/board/hisilicon/poplar/poplar.c
index 7794844..4926419 100644
--- a/board/hisilicon/poplar/poplar.c
+++ b/board/hisilicon/poplar/poplar.c
@@ -155,6 +155,7 @@
 }
 
 #if defined(CONFIG_USB_GADGET) && defined(CONFIG_USB_GADGET_DWC2_OTG)
+#include <env.h>
 #include <usb.h>
 #include <usb/dwc2_udc.h>
 #include <g_dnl.h>