Fix coverage remote

Remove boost and /nix/store stuff from the coverage report.

I think there is a better way of fixing the boost stuf using
geninfo_adjust_src_path, but I couldn't get it to work.

Author: roconnor-blockstream

elements-lcov.patch

@@ -0,0 +1,11 @@
+--- elements/Makefile.am
++++ elements/Makefile.am
+@@ -188,6 +188,8 @@
+ 	-p "/usr/include/" \
+ 	-p "/usr/lib/" \
+ 	-p "/usr/lib64/" \
++	-p "/nix/store/" \
++	-p "src/include/boost/" \
+ 	-p "src/leveldb/" \
+ 	-p "src/crc32c/" \
+ 	-p "src/bench/" \

elements.nix

@@ -33,6 +33,8 @@ stdenv.mkDerivation rec {
           sha256 = "sha256-UNjYkEZBjGuhkwBxSkNXjBBcLQqoan/afCLhoR2lOY4=";
         };
 
+  patches = [ ./elements-lcov.patch ];
+
   postPatch = optionals (doCheck)
   ''
     patchShebangs contrib/filter-lcov.py