wireless-regdb: prefer python provided by make variable

Usage of predefined make variables is preferred.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit d3853d17a37a67541736b8b13f412a49ab382b9a)

package/firmware/wireless-regdb/Makefile

@@ -19,7 +19,7 @@ define Package/wireless-regdb
 endef
 
 define Build/Compile
-	python $(PKG_BUILD_DIR)/db2fw.py $(PKG_BUILD_DIR)/regulatory.db $(PKG_BUILD_DIR)/db.txt
+	$(PYTHON) $(PKG_BUILD_DIR)/db2fw.py $(PKG_BUILD_DIR)/regulatory.db $(PKG_BUILD_DIR)/db.txt
 endef
 
 define Package/wireless-regdb/install