lua: create .gitkeep files in empty directories

Directories /lib/lua/5.3/ and /share/lua/5.3/ can be used by Lua modules
installed by luarocks, for example.

See https://gist.github.com/tonytheodore/4815252165e0f7eae740#file-build-pkg-only-deps2-L1350

src/lua.mk

@@ -20,6 +20,9 @@ define $(PKG)_UPDATE
 endef
 
 define $(PKG)_BUILD_COMMON
+    touch '$(PREFIX)/$(TARGET)/lib/lua/$($(PKG)_SHORTVER)/.gitkeep'
+    touch '$(PREFIX)/$(TARGET)/share/lua/$($(PKG)_SHORTVER)/.gitkeep'
+
     #pkg-config file
     (echo 'Name: $(PKG)'; \
      echo 'Version: $($(PKG)_VERSION)'; \