package/pkg-waf: add missing $

We want to have $(@D) expanded at the time the recipe is run, so like
all other variables, we need to $$-expand it.

Fixes: 1b4d7f6e13212b4c65ec2832681719fd960e75e6
Fixes: http://autobuild.buildroot.org/results/b6f/b6fd3a866af182edc7831492aecc8323f377b826

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-waf.mk

@@ -42,7 +42,7 @@ define inner-waf-package
 # otherwise waf errors out with:
 # Waf script 'X' and library 'Y' do not match
 define WAF_PACKAGE_REMOVE_WAF_LIB
-	$(RM) -fr $(@D)/waf $(@D)/waflib
+	$(RM) -fr $$(@D)/waf $$(@D)/waflib
 endef
 
 # We need host-python3 to run waf