libepoxy: propagate buildInputs

Some packages that depend on libepoxy fail to build due to libGL and
libX11 not being present during the build. One example is `qemu_xen`
(or really any QEMU built with GL support), as it's built with libepoxy,
which links against libGL, but since libGL isn't present, the build fails.

Signed-off-by: Fernando Rodrigues <alpha@sigmasquadron.net>

pkgs/by-name/li/libepoxy/package.nix

@@ -55,7 +55,7 @@ stdenv.mkDerivation (finalAttrs: {
     python3
   ];
 
-  buildInputs =
+  propagatedBuildInputs =
     lib.optionals (x11Support && !stdenv.hostPlatform.isDarwin) [
       libGL
     ]