package/pkg-waf: add one more missing $

Further fix on top on 9d1b223b91b0 (package/pkg-waf: add missing $).

Repeat after me: all variables in an inner-package macro must be
expanded, except for: parameters, pkgdir, and pkgname.

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