sasquatch: drop blanket -Werror (fix gcc-11 build) (#139350)

Noticed build failure on gcc-11:

$ nix-build -E 'with import ./.{}; sasquatch.override { stdenv = gcc11Stdenv; }'
unsquashfs.c:1908:5:
error: this 'if' clause does not guard... [-Werror=misleading-indentation]
1908 | if(swap)
| ^~

Let's defer warning squashing and code fixes to upstream.

pkgs/tools/filesystems/sasquatch/default.nix

···
   patchFlags = [ "-p0" ];
 
   postPatch = ''
+    # Drop blanket -Werror to avoid build failure on fresh toolchains
+    # like gcc-11.
+    substituteInPlace squashfs-tools/Makefile --replace ' -Werror' ' '
     cd squashfs-tools
   '';