guilt-native: eliminate hardcoded "/usr" in do_install

For some reason guilt-native seems to have gone out of its way to refer explicitly to /usr, which breaks on micro.
Let's use ${prefix} instead.

(From OE-Core rev: e3ada06e13820539425a4780cd749bf974b96b6f)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/guilt/guilt-native_0.33.bb

@@ -28,5 +28,5 @@ do_compile() {
 }
 
 do_install() {
-	oe_runmake PREFIX=${D}/${base_prefix}/usr install
+	oe_runmake PREFIX=${D}/${prefix} install
 }