gn: Allow use in SDK

It is obviously convenient to be able to provide an SDK with GN support, for a
one-stop solution to offering projects such as OpenHarmony with a replacement
toolchain.

Signed-off-by: Esben Haabendal <esben.haabendal@huawei.com>

meta-oniro-staging/recipes-devtools/gn/gn_git.bb

@@ -44,7 +44,7 @@ do_install() {
     install ${B}/gn ${D}${bindir}
 }
 
-BBCLASSEXTEND = "native"
+BBCLASSEXTEND = "native nativesdk"
 
 COMPATIBLE_HOST = "^(?!riscv32).*"