pangomm: fix build for v 2.42.2

The build of `pangomm` 2.42 is broken due to a missing include statement
in `attrlist.cc` (a generated source file).

Others have already run into this issue and @dslm4515 has outlined a
patch in a respective issue on their BMLFS project [1].

[1]: https://github.com/dslm4515/BMLFS/issues/16#issuecomment-914624797

pkgs/development/libraries/pangomm/2.42.2-add-missing-include-attrlist.cc.patch

@@ -0,0 +1,12 @@
+diff --git a/untracked/pango/pangomm/attrlist.cc b/untracked/pango/pangomm/attrlist.cc
+index 76c3341..0c6d68d 100755
+--- a/untracked/pango/pangomm/attrlist.cc
++++ b/untracked/pango/pangomm/attrlist.cc
+@@ -3,6 +3,7 @@
+ 
+ #include <glibmm.h>
+ 
++#include <pango-1.0/pango/pango-markup.h>
+ #include <pangomm/attrlist.h>
+ #include <pangomm/private/attrlist_p.h>
+ 

pkgs/development/libraries/pangomm/2.42.nix

@@ -10,6 +10,15 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-GyTJJiSuEnXMtXdYF10198Oa0zQtjAtLpg8NmEnS0Io=";
   };
 
+  patches = [
+    # Fixes a missing include leading to build failures while compiling `attrlist.cc` (as outlined by @dslm4515 [1])
+    # Note that the files in that directory are generated and not tracked in Git [2], which is why we can't simply
+    # try to cherry-pick an upstream patch from future versions.
+    # [1]: https://github.com/dslm4515/BMLFS/issues/16#issuecomment-914624797
+    # [2]: https://github.com/GNOME/pangomm/tree/master/untracked
+    ./2.42.2-add-missing-include-attrlist.cc.patch
+  ];
+
   outputs = [ "out" "dev" ];
 
   nativeBuildInputs = [ pkg-config meson ninja python3 ] ++ lib.optionals stdenv.isDarwin [