Add missing make dependency

Recently, Ruby stopped shipping RDoc by default. For this reason, users
without an explicit RDoc installation would face building issues with
this PKGBUILD. To fix this, this commit adds the now missing ruby-rdoc
make dependency.

.SRCINFO

@@ -1,10 +1,11 @@
 pkgbase = ruby-liquid
 	pkgdesc = Liquid markup language. Safe, customer facing template language for flexible web apps
 	pkgver = 4.0.0
-	pkgrel = 3
+	pkgrel = 4
 	url = https://shopify.github.io/liquid/
 	arch = any
 	license = MIT
+	makedepends = ruby-rdoc
 	depends = ruby
 	options = !emptydirs
 	source = ruby-liquid-4.0.0.tar.gz::https://github.com/Shopify/liquid/archive/v4.0.0.tar.gz

PKGBUILD

@@ -3,11 +3,12 @@
 _gemname=liquid
 pkgname=ruby-$_gemname
 pkgver=4.0.0
-pkgrel=3
+pkgrel=4
 pkgdesc='Liquid markup language. Safe, customer facing template language for flexible web apps'
 url='https://shopify.github.io/liquid/'
 arch=('any')
 license=('MIT')
+makedepends=('ruby-rdoc')
 depends=('ruby')
 options=('!emptydirs')
 source=(${pkgname}-${pkgver}.tar.gz::https://github.com/Shopify/${_gemname}/archive/v${pkgver}.tar.gz)