gdb: Do not link against xxhash

libxxhash is now available in the OpenWrt package feed and gdb will link
against it if gdb finds this library. Explicitly deactivate the usage
of xxhash.

This should fix the build of gdb in build bots.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/devel/gdb/Makefile

@@ -66,6 +66,7 @@ CONFIGURE_ARGS+= \
 	--without-mpc \
 	--without-mpfr \
 	--without-isl \
+	--without-xxhash \
 	--with-libgmp-prefix=$(STAGING_DIR)/usr
 
 CONFIGURE_VARS+= \