gstreamer: make --enable-introspection=yes explicitly

Closes Homebrew/homebrew#28821.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/gstreamer.rb

@@ -1,23 +1,23 @@
-require 'formula'
+require "formula"
 
 class Gstreamer < Formula
-  homepage 'http://gstreamer.freedesktop.org/'
-  url 'http://gstreamer.freedesktop.org/src/gstreamer/gstreamer-1.2.4.tar.xz'
-  mirror 'http://ftp.osuosl.org/pub/blfs/svn/g/gstreamer-1.2.4.tar.xz'
-  sha256 '1e7ca67a7870a82c9ed51d51d0008cdbc550c41d64cc3ff3f9a1c2fc311b4929'
+  homepage "http://gstreamer.freedesktop.org/"
+  url "http://gstreamer.freedesktop.org/src/gstreamer/gstreamer-1.2.4.tar.xz"
+  mirror "http://ftp.osuosl.org/pub/blfs/svn/g/gstreamer-1.2.4.tar.xz"
+  sha256 "1e7ca67a7870a82c9ed51d51d0008cdbc550c41d64cc3ff3f9a1c2fc311b4929"
 
   head do
-    url 'git://anongit.freedesktop.org/gstreamer/gstreamer'
+    url "git://anongit.freedesktop.org/gstreamer/gstreamer"
 
     depends_on :autoconf
     depends_on :automake
     depends_on :libtool
   end
 
-  depends_on 'pkg-config' => :build
-  depends_on 'gobject-introspection' => :optional
-  depends_on 'gettext'
-  depends_on 'glib'
+  depends_on "pkg-config" => :build
+  depends_on "gobject-introspection"
+  depends_on "gettext"
+  depends_on "glib"
 
   def install
     ENV.append "CFLAGS", "-funroll-loops -fstrict-aliasing -fno-common"
@@ -27,6 +27,7 @@ class Gstreamer < Formula
       --disable-debug
       --disable-dependency-tracking
       --disable-gtk-doc
+      --enable-introspection=yes
     ]
 
     if build.head?