Use /usr/bin in BuildRequires rather than %_bindir

When building flatpaks, using %_bindir in BRs breaks the build as it
then points to /app.

simple-scan.spec

@@ -11,9 +11,9 @@ BuildRequires:  meson
 BuildRequires:  sane-backends-devel
 BuildRequires:  gettext
 BuildRequires:  itstool
-BuildRequires:  %{_bindir}/appstream-util
+BuildRequires:  /usr/bin/appstream-util
-BuildRequires:  %{_bindir}/desktop-file-validate
+BuildRequires:  /usr/bin/desktop-file-validate
-BuildRequires:  %{_bindir}/valac
+BuildRequires:  /usr/bin/valac
 BuildRequires:  pkgconfig(colord)
 BuildRequires:  pkgconfig(gtk+-3.0)
 BuildRequires:  pkgconfig(gudev-1.0)