Fix the paxmarking of the xpcshell binary as it is no longer in the mozilla
directory.

This makes the install phase work again on hardened systems.

Signed-Off-By: Francisco Blas Izquierdo Riera (klondike) <klondike@gentoo.org>

--- /usr/portage/mail-client/thunderbird/thunderbird-38.5.0.ebuild
+++ /usr/portage/mail-client/thunderbird/thunderbird-38.5.0-r1.ebuild
@@ -284,7 +284,7 @@
 		|| die
 
 	# Pax mark xpcshell for hardened support, only used for startupcache creation.
-	pax-mark m "${BUILD_OBJ_DIR}"/mozilla/dist/bin/xpcshell
+	pax-mark m "${BUILD_OBJ_DIR}"/dist/bin/xpcshell
 
 	MOZ_MAKE_FLAGS="${MAKEOPTS}" \
 	emake DESTDIR="${D}" install