libargtable2: fix source tarball name

Last minute brown paper bag bug.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libargtable2/libargtable2.mk

@@ -5,6 +5,7 @@
 #############################################################
 
 LIBARGTABLE2_VERSION = 13
+LIBARGTABLE2_SOURCE = argtable2-$(LIBARGTABLE2_VERSION).tar.gz
 LIBARGTABLE2_SITE = http://$(BR2_SOURCEFORGE_MIRROR).dl.sourceforge.net/sourceforge/argtable/
 
 LIBARGTABLE2_INSTALL_STAGING = YES