dpkg: symlink pkg-config file to default location

dpkg includes a pkg-config file -- it works if I manually specified
PKG_CONFIG_PATH=$(brew --prefix dpkg)/libexec/lib/pkgconfig so this
fixes it to install the .pc file into the right default search path.

Closes #1317.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/dpkg.rb

@@ -40,6 +40,7 @@ def install
 
     bin.install Dir["#{libexec}/bin/*"]
     man.install Dir["#{libexec}/share/man/*"]
+    (lib/"pkgconfig").install_symlink Dir["#{libexec}/lib/pkgconfig/*.pc"]
     bin.env_script_all_files(libexec+"bin", :PERL5LIB => ENV["PERL5LIB"])
 
     (buildpath/"dummy").write "Vendor: dummy\n"