package/luajit: fix LUAJIT_XCFLAGS

Fix the following build failure raised since commit
d4e6e1b53b07b05d51ebfb70979608c144ac155a:

/usr/bin/make: invalid option -- 'D'
/usr/bin/make: invalid option -- 'U'
/usr/bin/make: invalid option -- 'A'
/usr/bin/make: invalid option -- 'J'

Fixes:
 - http://autobuild.buildroot.org/results/92a5188ef7f9dda3e417d3853b0c4067415f5970

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/luajit/luajit.mk

@@ -47,7 +47,7 @@ define LUAJIT_BUILD_CMDS
 		HOST_CFLAGS="$(HOST_CFLAGS)" \
 		HOST_LDFLAGS="$(HOST_LDFLAGS)" \
 		BUILDMODE=dynamic \
-		XCFLAGS=$(LUAJIT_XCFLAGS) \
+		XCFLAGS="$(LUAJIT_XCFLAGS)" \
 		-C $(@D) amalg
 endef