lttng-ust: Fix case where ${B} != ${S}

Fix out of tree builds by ensuring bootstrap is executed in ${S}.

(From OE-Core rev: 970be6b43acd16dac1400ce322dcd2cb6272fd63)

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

meta/recipes-kernel/lttng/lttng-ust_2.1.1.bb

@@ -29,7 +29,7 @@ SRC_URI = "git://git.lttng.org/lttng-ust.git;protocol=git \
 S = "${WORKDIR}/git"
 
 do_configure_prepend () {
-	${S}/bootstrap
+	( cd ${S}; ${S}/bootstrap )
 }
 
 # Due to liburcu not building for MIPS currently this recipe needs to