[source] wireguard: move to kernel build directory

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/f7186599ce72bfcc6f6442c9f43fa65f76d68206

commit f7186599ce72bfcc6f6442c9f43fa65f76d68206
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Sat Nov 11 13:01:50 2017 +0100

    wireguard: move to kernel build directory
    
    It builds a kernel module, so its build dir should be target specific
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 package/network/services/wireguard/Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/package/network/services/wireguard/Makefile b/package/network/services/wireguard/Makefile
index f9ed6ba..9be2a4c 100644
--- a/package/network/services/wireguard/Makefile
+++ b/package/network/services/wireguard/Makefile
@@ -21,7 +21,7 @@ PKG_HASH:=096b6482a65e566c7bf8c059f5ee6aadb2de565b04b6d810c685f1c377540325
 PKG_LICENSE:=GPL-2.0 Apache-2.0
 PKG_LICENSE_FILES:=COPYING
 
-PKG_BUILD_DIR:=$(BUILD_DIR)/WireGuard-$(PKG_VERSION)
+PKG_BUILD_DIR:=$(KERNEL_BUILD_DIR)/WireGuard-$(PKG_VERSION)
 PKG_BUILD_PARALLEL:=1
 PKG_USE_MIPS16:=0