package/xen: add slirp dependency for tools

Build of xen tools fails if slirp is built before xen because xen is not
compatible with spice slirp which does not provide libslirp.h:

/home/buildroot/autobuild/instance-2/output-1/build/xen-4.13.0/tools/qemu-xen/net/slirp.c:40:10: fatal error: libslirp.h: No such file or directory
 #include <libslirp.h>
          ^~~~~~~~~~~~

Indeed, xen prefers a system-provided slirp over its internal one

So add slirp as a mandatory dependency (now that we switched to the up
to date https://gitlab.freedesktop.org/slirp/libslirp)

This build failure is raised since, at least, version 4.13.0

Fixes:
 - http://autobuild.buildroot.org/results/b80b33ed558518f7bbb0a3c8586bf2d0b8acc36f

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
(cherry picked from commit a0a5c184efe4fec3bf7edb0a7a8e40bb4ddbea13)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/xen/Config.in

@@ -28,6 +28,7 @@ config BR2_PACKAGE_XEN_TOOLS
 	select BR2_PACKAGE_NCURSES
 	select BR2_PACKAGE_OPENSSL
 	select BR2_PACKAGE_PIXMAN
+	select BR2_PACKAGE_SLIRP
 	select BR2_PACKAGE_UTIL_LINUX
 	select BR2_PACKAGE_UTIL_LINUX_LIBUUID
 	select BR2_PACKAGE_YAJL

package/xen/xen.mk

@@ -42,7 +42,8 @@ XEN_CONF_OPTS += --disable-xen
 endif
 
 ifeq ($(BR2_PACKAGE_XEN_TOOLS),y)
-XEN_DEPENDENCIES += dtc libaio libglib2 ncurses openssl pixman util-linux yajl
+XEN_DEPENDENCIES += \
+	dtc libaio libglib2 ncurses openssl pixman slirp util-linux yajl
 ifeq ($(BR2_PACKAGE_ARGP_STANDALONE),y)
 XEN_DEPENDENCIES += argp-standalone
 endif