Adjust hack to be more friendly to makepkg

This keeps the same benefits of the original solution, but is more compatible
with makepkg's extraction methods.

PKGBUILD

@@ -288,7 +288,7 @@ _package-docs() {
 pkgname=("${pkgbase}" "${pkgbase}-headers" "${pkgbase}-docs")
 for _p in ${pkgname[@]}; do
   eval "package_${_p}() {
-    _package${_p#${pkgbase}}
+    $(declare -f "_package${_p#${pkgbase}}")
   }"
 done