jpeg: fix make source

'make source' fails because the host-jpeg-source target doesn't exist
anymore. Fix this by adding this target explicitly.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/jpeg/jpeg.mk

@@ -7,3 +7,4 @@
 jpeg: $(if $(BR2_PACKAGE_JPEG_TURBO),jpeg-turbo,libjpeg)
 
 host-jpeg: host-libjpeg
+host-jpeg-source: host-libjpeg-source