qt5: fix pkg-config

Commit 77fd56f4a9 ("qt5: remove spurious
pkg-config dependency") dropped the pkg-config build dependency because the
missing ./configure --pkg-config option caused Qt to be built without
pkg-config support.

pkg-config makes it easy to link Qt applications against external
libraries.  Many libraries available in Homebrew ship with pkg-config .pc
files so it makes sense to enable this feature.

This patch re-enables pkg-config so qmake CONFIG += link_pkgconfig works as
originally intended in commit b4df7fce96791c72e8fb4c84a89258eea990117a ("qt5:
add pkg-config build dependency").

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

Closes #7134.

Signed-off-by: ilovezfs <ilovezfs@icloud.com>

Formula/qt5.rb

@@ -99,6 +99,7 @@ class Qt5 < Formula
 
   depends_on "dbus" => :optional
   depends_on :mysql => :optional
+  depends_on "pkg-config" => :build
   depends_on :postgresql => :optional
   depends_on :xcode => :build
 
@@ -136,6 +137,7 @@ class Qt5 < Formula
       -qt-pcre
       -nomake tests
       -no-rpath
+      -pkg-config
     ]
 
     args << "-nomake" << "examples" if build.without? "examples"